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Table of Contents
INDEX TO FINANCIAL STATEMENTS
TABLE OF CONTENTS Prospectus Supplement
AUDITED FINANCIAL STATEMENTS

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Filed Pursuant to Rule 497
Registration Statement No. 333-183555

The information in this preliminary prospectus supplement is not complete and may be changed. A registration statement relating to these securities has been filed with and declared effective by the Securities and Exchange Commission. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell and are not soliciting offers to buy these securities in any state where such offer or sale is not permitted.

SUBJECT TO COMPLETION, DATED MARCH 26, 2013

PRELIMINARY PROSPECTUS SUPPLEMENT
(to Prospectus dated October 19, 2012)

Main Street Capital Corporation

$            
            % Senior Notes due 2023

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             We are offering $            in aggregate principal amount of        % senior notes due 2023, which we refer to as the Notes. The Notes will mature on April 1, 2023. We will pay interest on the Notes on January 1, April 1, July 1 and October 1 of each year, beginning July 1, 2013. In our sole discretion, we may redeem the Notes in whole or in part at any time or from time to time on or after April 1, 2018, at the redemption price of par, plus accrued interest, as discussed under the caption "Specific Terms of the Notes and the Offering—Optional redemption" in this prospectus supplement. The Notes will be issued in minimum denominations of $25 and integral multiples of $25 in excess thereof.

             The Notes will be our direct senior unsecured obligations and rank pari passu with all outstanding and future unsecured unsubordinated indebtedness issued by Main Street Capital Corporation.

             We intend to list the Notes on the New York Stock Exchange and we expect trading to commence thereon within 30 days of the original issue date under the trading symbol "MSCA." The Notes are expected to trade "flat." This means that purchasers will not pay, and sellers will not receive, any accrued and unpaid interest on the Notes that is not included in the trading price. Currently, there is no public market for the Notes.

             We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market ("LMM") companies and debt capital to middle market ("Middle Market") companies. Our LMM companies generally have annual revenues between $10 million and $150 million, and our LMM portfolio investments generally range in size from $5 million to $25 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies, with annual revenues typically between $150 million and $1.5 billion, and our Middle Market investments generally range in size from $3 million to $15 million.

             The LMM and Middle Market securities in which we invest generally would be rated below investment grade if they were rated by rating agencies. Below investment grade securities, which are often referred to as "junk," have predominantly speculative characteristics with respect to the issuer's capacity to pay interest and repay principal. They may also be difficult to value and are illiquid.

             Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company.

             We are an internally managed, closed-end, non-diversified management investment company that has elected to be treated as a business development company under the Investment Company Act of 1940.

             This prospectus supplement and the accompanying prospectus contain important information about us that a prospective investor should know before investing in our Notes. Please read this prospectus supplement and the accompanying prospectus before investing and keep them for future reference. We file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission, or SEC. This information is available free of charge by contacting us at 1300 Post Oak Boulevard, Suite 800, Houston, Texas 77056 or by telephone at (713) 350-6000 or on our website at www.mainstcapital.com. Information contained on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus, and you should not consider that information to be part of this prospectus supplement or the accompanying prospectus. The SEC also maintains a website at www.sec.gov that contains such information.

             Investing in the Notes involves a high degree of risk and should be considered highly speculative. See "Supplementary Risk Factors" beginning on page S-12 of this prospectus supplement and "Risk Factors" on page 13 of the accompanying prospectus to read about factors you should consider, including the risk of leverage, before investing in the Notes.

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We estimate that we will incur approximately $200,000 in offering expenses in connection with this offering.

             The underwriters may also purchase up to an additional $            aggregate principal amount of Notes offered hereby, to cover over-allotments, if any, within 30 days of the date of this prospectus supplement. If the underwriters exercise this option in full, the total public offering price will be $            , the total underwriting discount (sales load) paid by us will be $            , and total proceeds, before expenses, will be $      .

             The public offering price set forth above does not include accrued interest, if any. Interest on the Notes will accrue from April     , 2013 and must be paid by the purchaser if the Notes are delivered after April     , 2013.

             THE NOTES ARE NOT DEPOSITS OR OTHER OBLIGATIONS OF A BANK AND ARE NOT INSURED BY THE FEDERAL DEPOSIT INSURANCE CORPORATION OR ANY OTHER GOVERNMENT AGENCY.

             The Securities and Exchange Commission has not approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

             Delivery of the Notes in book-entry form only through The Depository Trust Company will be made on or about April     , 2013.

Joint Book-Running Managers

Lead Co-Managers

Co-Managers

   

The date of this prospectus supplement is March     , 2013.

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TABLE OF CONTENTS

Prospectus Supplement

Prospectus

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ABOUT THE PROSPECTUS

        This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering of Notes and also adds to and updates information contained in the accompanying prospectus. The second part is the accompanying prospectus, which provides more information about the securities we may offer from time to time. To the extent the information contained in this prospectus supplement differs from the information contained in the accompanying prospectus, the information in this prospectus supplement shall control.

        You should rely only on the information contained in this prospectus supplement and the accompanying prospectus. Neither we nor the underwriters have authorized any other person to provide you with different information from that contained in this prospectus supplement or the accompanying prospectus. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement and the accompanying prospectus do not constitute an offer to sell, or a solicitation of an offer to buy, any Notes by any person in any jurisdiction where it is unlawful for that person to make such an offer or solicitation or to any person in any jurisdiction to whom it is unlawful to make such an offer or solicitation. The information contained in this prospectus supplement and the accompanying prospectus is complete and accurate only as of their respective dates, regardless of the time of their delivery or sale of our Notes. This prospectus supplement supersedes the accompanying prospectus to the extent it contains information different from or additional to the information in that prospectus.

Forward-Looking Statements

        Information contained in this prospectus supplement and the accompanying prospectus may contain forward-looking statements, which can be identified by the use of forward-looking terminology such as "may," "will," "expect," "intend," "anticipate," "estimate," or "continue" or the negative thereof or other variations thereon or comparable terminology. The matters described in the sections titled "Supplementary Risk Factors" in this prospectus supplement and "Risk Factors" in the accompanying prospectus and certain other factors noted throughout this prospectus supplement and the accompanying prospectus constitute cautionary statements identifying important factors with respect to any such forward-looking statements, including certain risks and uncertainties that could cause actual results to differ materially from those in such forward-looking statements. We undertake no obligation to revise or update any forward-looking statements but advise you to consult any additional disclosures that we may make directly to you or through reports that we may file in the future with the SEC, including annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K. We note that the safe harbor for forward-looking statements provided by the Private Securities Litigation Reform Act of 1995 does not apply to statements made in this prospectus supplement or the accompanying prospectus.

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SUPPLEMENTARY RISK FACTORS

        Investing in the Notes involves a number of significant risks. In addition to the other information contained in this prospectus supplement and the accompanying prospectus, you should carefully consider the following supplementary risk factors together with the risk factors set forth in the accompanying prospectus before making an investment in the Notes. The risks set out below and in the accompanying prospectus are not the only risks we face. Additional risks and uncertainties not presently known to us or not presently deemed material by us might also impair our operations and performance. If any of the events described herein or in the accompanying prospectus occur, our business, financial condition and results of operations could be materially and adversely affected. In such case, the market price of the Notes could decline, and you may lose part or all of your investment.

Risks Relating to the Notes

The Notes will be unsecured and therefore will be effectively subordinated to any current or future secured indebtedness, including indebtedness under the Credit Facility.

        The Notes will not be secured by any of our assets or any of the assets of our subsidiaries and will rank equally in right of payment with all of our existing and future unsubordinated, unsecured senior indebtedness. As a result, the Notes are effectively subordinated to any secured indebtedness we or our subsidiaries have currently incurred and may incur in the future (or any indebtedness that is initially unsecured to which we subsequently grant security) to the extent of the value of the assets securing such indebtedness. In any liquidation, dissolution, bankruptcy or other similar proceeding, the holders of any of our existing or future secured indebtedness and the secured indebtedness of our subsidiaries may assert rights against the assets pledged to secure that indebtedness in order to receive full payment of their indebtedness before the assets may be used to pay other creditors, including the holders of the Notes. As of March 25, 2013 we had $141.0 million outstanding under the Credit Facility and have the current ability to borrow up to $287.5 million under the Credit Facility, subject to certain conditions. The indebtedness under the Credit Facility is senior to the Notes to the extent of the value of the assets securing such indebtedness.

The Notes will be structurally subordinated to the indebtedness and other liabilities of our subsidiaries.

        The Notes are obligations exclusively of Main Street Capital Corporation and not of any of our subsidiaries. None of our subsidiaries is a guarantor of the Notes and the Notes are not required to be guaranteed by any subsidiaries we may acquire or create in the future. A significant portion of the indebtedness required to be consolidated on our balance sheet is held through our SBIC subsidiaries. For example, as of March 25, 2013, the Funds had collectively issued the current statutory maximum of $225.0 million of SBA-guaranteed debentures. The assets of such subsidiaries are not directly available to satisfy the claims of our creditors, including holders of the Notes. See "Management's Discussion and Analysis of Financial Condition and Results of Operations—Liquidity and Capital Resources" in this prospectus supplement for more detail on the SBA-guaranteed debentures.

        Except to the extent we are a creditor with recognized claims against our subsidiaries, all claims of other creditors of our subsidiaries will have priority over our equity interests in such subsidiaries (and therefore the claims of our creditors, including holders of the Notes) with respect to the assets of such subsidiaries. Even if we are recognized as a creditor of one or more of our subsidiaries, our claims would still be effectively subordinated to any security interests in the assets of any such subsidiary and to any indebtedness or other liabilities of any such subsidiary senior to our claims. Consequently, the Notes will be structurally subordinated to all indebtedness, including the SBA-guaranteed debentures, and other liabilities of any of our subsidiaries and any subsidiaries that we may in the future acquire or establish. In addition, our subsidiaries may incur substantial additional indebtedness in the future, all of which would be structurally senior to the Notes.

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The indenture under which the Notes will be issued contains limited protection for holders of the Notes.

        The indenture under which the Notes will be issued offers limited protection to holders of the Notes. The terms of the indenture and the Notes do not restrict our or any of our subsidiaries' ability to engage in, or otherwise be a party to, a variety of corporate transactions, circumstances or events that could have an adverse impact on your investment in the Notes. In particular, the terms of the indenture and the Notes will not place any restrictions on our or our subsidiaries' ability to:

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issue securities or otherwise incur additional indebtedness or other obligations, including (1) any indebtedness or other obligations that would be equal in right of payment to the Notes, (2) any indebtedness or other obligations that would be secured and therefore rank effectively senior in right of payment to the Notes to the extent of the values of the assets securing such debt, (3) indebtedness of ours that is guaranteed by one or more of our subsidiaries and which therefore is structurally senior to the Notes and (4) securities, indebtedness or obligations issued or incurred by our subsidiaries that would be senior to our equity interests in our subsidiaries and therefore rank structurally senior to the Notes with respect to the assets of our subsidiaries, in each case other than an incurrence of indebtedness or other obligation that would cause a violation of Section 18(a)(1)(A) as modified by Section 61(a)(1) of the 1940 Act or any successor provisions, but giving effect, in each case, to any exemptive relief granted to us by the SEC (currently, this provision generally prohibits us from making additional borrowings, including through the issuance of additional debt or the sale of additional debt securities, unless our asset coverage, as defined in the 1940 Act, equals at least 200% after such borrowings);



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pay dividends on, or purchase or redeem or make any payments in respect of, capital stock or other securities ranking junior in right of payment to the Notes, including subordinated indebtedness;



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sell assets (other than certain limited restrictions on our ability to consolidate, merge or sell all or substantially all of our assets);



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enter into transactions with affiliates;



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create liens (including liens on the shares of our subsidiaries) or enter into sale and leaseback transactions;



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make investments; or



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create restrictions on the payment of dividends or other amounts to us from our subsidiaries.

        In addition, the indenture will not require us to offer to purchase the Notes in connection with a change of control or any other event.

        Furthermore, the terms of the indenture and the Notes do not protect holders of the Notes in the event that we experience changes (including significant adverse changes) in our financial condition, results of operations or credit ratings, if any, as they do not require that we or our subsidiaries adhere to any financial tests or ratios or specified levels of net worth, revenues, income, cash flow, or liquidity other than as described under "Specific Terms of the Notes and the Offering—Events of default."

        Our ability to recapitalize, incur additional debt and take a number of other actions that are not limited by the terms of the Notes may have important consequences for you as a holder of the Notes, including making it more difficult for us to satisfy our obligations with respect to the Notes or negatively affecting the trading value of the Notes.

        Other debt we issue or incur in the future could contain more protections for its holders than the indenture and the Notes, including additional covenants and events of default. For example, the indenture under which the Notes will be issued does not contain cross-default provisions that are

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contained in the Credit Facility. The issuance or incurrence of any such debt with incremental protections could affect the market for and trading levels and prices of the Notes.

The Notes may not be approved for listing on the NYSE and an active trading market for the Notes may not develop, which could limit the market price of the Notes or your ability to sell them.

        The Notes are a new issue of debt securities for which there currently is no trading market. We intend to list the Notes on the NYSE within 30 days of the original issue date under the symbol "MSCA." Although we expect the Notes to be listed on the NYSE, we cannot provide any assurances that the Notes will be approved for listing, that an active trading market will develop or be maintained for the Notes or that you will be able to sell your Notes. If the Notes are traded after their initial issuance, they may trade at a discount from their initial offering price depending on prevailing interest rates, the market for similar securities, our credit ratings, if any, general economic conditions, our financial condition, performance and prospects and other factors. The underwriters have advised us that they intend to make a market in the Notes, but they are not obligated to do so. The underwriters may discontinue any market-making in the Notes at any time at their sole discretion. Accordingly, we cannot assure you that a liquid trading market will develop for the Notes, that you will be able to sell your Notes at a particular time or that the price you receive when you sell will be favorable. To the extent an active trading market does not develop, the liquidity and trading price for the Notes may be harmed. Accordingly, you may be required to bear the financial risk of an investment in the Notes for an indefinite period of time.

We may choose to redeem the Notes when prevailing interest rates are relatively low.

        On or after April 1, 2018, we may choose to redeem the Notes from time to time, especially when prevailing interest rates are lower than the rate borne by the Notes. If prevailing rates are lower at the time of redemption, you would not be able to reinvest the redemption proceeds in a comparable security at an effective interest rate as high as the interest rate on the Notes being redeemed. Our redemption right also may adversely impact your ability to sell the Notes as the optional redemption date or period approaches.

Our amount of debt outstanding will increase as a result of this offering, and if we default on our obligations to pay our other indebtedness, we may not be able to make payments on the Notes.

        As of March 25, 2013, we had approximately $366.0 million of indebtedness, including $141.0 million outstanding under the Credit Facility and $225.0 million outstanding from SBA-guaranteed debentures. Any default under the agreements governing our indebtedness, including a default under the Credit Facility or other indebtedness to which we may be a party that is not waived by the required lenders, and the remedies sought by the holders of such indebtedness could make us unable to pay principal, premium, if any, and interest on the Notes and substantially decrease the market value of the Notes. If we are unable to generate sufficient cash flow and are otherwise unable to obtain funds necessary to meet required payments of principal, premium, if any, and interest on our indebtedness, or if we otherwise fail to comply with the various covenants, including financial and operating covenants, in the instruments governing our indebtedness (including the Credit Facility), we could be in default under the terms of the agreements governing such indebtedness. In the event of such default, the holders of such indebtedness could elect to declare all the funds borrowed thereunder to be due and payable, together with accrued and unpaid interest, the lenders under the Credit Facility or other debt we may incur in the future could elect to terminate their commitments, cease making further loans and institute foreclosure proceedings against our assets, and we could be forced into bankruptcy or liquidation. Our ability to generate sufficient cash flow in the future is, to some extent, subject to general economic, financial, competitive, legislative and regulatory factors as well as other factors that are beyond our control. We cannot assure you that our business will generate cash flow

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from operations, or that future borrowings will be available to us under the Credit Facility or otherwise, in an amount sufficient to enable us to meet our payment obligations under the Notes and our other debt and to fund other liquidity needs.

        If our operating performance declines and we are not able to generate sufficient cash flow to service our debt obligations, we may in the future need to refinance or restructure our debt, including any Notes sold, sell assets, reduce or delay capital investments, seek to raise additional capital or seek to obtain waivers from the required lenders under the Credit Facility or other debt that we may incur in the future to avoid being in default. If we are unable to implement one or more of these alternatives, we may not be able to meet our payment obligations under the Notes and our other debt. If we breach our covenants under the Credit Facility or other debt and seek a waiver, we may not be able to obtain a waiver from the required lenders. If this occurs, we would be in default under the Credit Facility or other debt, the lenders could exercise their rights as described above, and we could be forced into bankruptcy or liquidation. If we are unable to repay debt, lenders having secured obligations could proceed against the collateral securing the debt. Because the Credit Facility has, and any future credit facilities will likely have, customary cross-default provisions, if the indebtedness under the Notes, the Credit Facility or under any future credit facility is accelerated, we may be unable to repay or finance the amounts due.

Risks Relating to Economic Conditions

Deterioration in the economy and financial markets increases the likelihood of adverse effects on our financial position and results of operations. Such economic adversity could impair our portfolio companies' financial positions and operating results and affect the industries in which we invest, which could, in turn, harm our operating results.

        As a result of the recent recession, the broader fundamentals of the United States economy remain mixed, and unemployment remains elevated. In the event that the United States economy contracts, it is likely that the financial results of small to mid-sized companies, like those in which we invest, could experience deterioration or limited growth from current levels, which could ultimately lead to difficulty in meeting their debt service requirements and an increase in defaults. Consequently, we can provide no assurance that the performance of certain portfolio companies will not be negatively impacted by economic cycles or other conditions, which could also have a negative impact on our future results.

        Although we have been able to secure access to additional liquidity, including through the Credit Facility, periodic follow-on equity offerings and the leverage available through the SBIC program, the potential for volatility in the debt and equity capital markets provides no assurance that debt or equity capital will be available to us in the future on favorable terms, or at all. Further, if the price of our common stock falls below our net asset value per share, we will be limited in our ability to sell new shares if we do not have shareholder authorization to sell shares at a price below net asset value per share. We have been authorized by our stockholders to sell shares of common stock at a price below our net asset value per share until the earlier of June 14, 2013 or our 2013 Annual Meeting of Stockholders; however, we do not intend to seek the extension of such shareholder authorization at our 2013 Annual Meeting of Stockholders.

Risks Relating to Our Business and Structure

Because we borrow money, the potential for gain or loss on amounts invested in us is magnified and may increase the risk of investing in us.

        Borrowings, also known as leverage, magnify the potential for loss on investments in our indebtedness and gain or loss on invested equity capital. As we use leverage to partially finance our investments, you will experience increased risks of investing in our securities. We, through the Funds,

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issue debt securities guaranteed by the SBA and sold in the capital markets. As a result of its guarantee of the debt securities, the SBA has fixed dollar claims on the assets of the Funds that are superior to the claims of our securities holders. We may also borrow from banks and other lenders, including under our Credit Facility, and may issue debt securities or enter into other types of borrowing arrangements in the future. See "Management's Discussion and Analysis of Financial Condition and Results of Operations—Capital Resources" for a discussion regarding our Credit Facility. If the value of our assets increases, then leveraging would cause the net asset value attributable to our common stock to increase more sharply than it would have had we not leveraged. Conversely, if the value of our assets decreases, leveraging would cause net asset value to decline more sharply than it otherwise would have had we not leveraged our business. Similarly, any increase in our income in excess of interest payable on the borrowed funds would cause our net investment income to increase more than it would without the leverage, while any decrease in our income would cause net investment income to decline more sharply than it would have had we not borrowed. Such a decline could negatively affect our ability to pay common stock dividends, scheduled debt payments or other payments related to our securities. Leverage is generally considered a speculative investment technique.

        As of December 31, 2012, we, through the Funds, had $225 million of outstanding indebtedness guaranteed by the SBA, which had a weighted average annualized interest cost of approximately 4.7% (exclusive of deferred financing costs). The debentures guaranteed by the SBA have a maturity of ten years, with a current weighted average remaining maturity of 6.4 years as of December 31, 2012, and require semi-annual payments of interest. We will need to generate sufficient cash flow to make required interest payments on the debentures. If we are unable to meet the financial obligations under the debentures, the SBA, as a creditor, will have a superior claim to the assets of the Funds over the holders of our other indebtedness (including the Notes) and our stockholders in the event we liquidate or the SBA exercises its remedies under such debentures as the result of a default by us. In addition, as of December 31, 2012, we had $132 million outstanding under our Credit Facility. Borrowings under the Credit Facility bear interest, subject to our election, on a per annum basis equal to (i) the applicable LIBOR rate (0.21% as of December 31, 2012) plus 2.50% or (ii) the applicable base rate (Prime Rate, 3.25% as of December 31, 2012) plus 1.50%. Main Street pays unused commitment fees of 0.375% per annum on the average unused lender commitments under the Credit Facility. If we are unable to meet the financial obligations under the Credit Facility, the Credit Facility lending group will have a superior claim to the assets of MSCC and its subsidiaries (excluding the assets of the Funds) over the holders of our other indebtedness (including the Notes) and our stockholders in the event we liquidate or the lending group exercises its remedies under the Credit Facility as the result of a default by us.

        Illustration.    The following table illustrates the effect of leverage on returns from an investment in our common stock assuming various annual returns, net of expenses. The calculations in the table below are hypothetical and actual returns may be higher or lower than those appearing below.

Assumed Return on Our Portfolio(1)

(net of expenses)

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        Our ability to achieve our investment objective may depend in part on our ability to access additional leverage on favorable terms by issuing debentures guaranteed by the SBA, through the Funds, or by borrowing from banks or insurance companies, and there can be no assurance that such additional leverage can in fact be achieved.

Pending legislation may allow us to incur additional leverage.

        As a BDC, under the 1940 Act we generally are not permitted to incur indebtedness unless immediately after such borrowing we have an asset coverage for total borrowings of at least 200% (i.e., the amount of debt may not exceed 50% of the value of our assets). We have agreed in a covenant in the indenture governing the Notes not to violate this section of the 1940 Act, whether or not we continue to be subject to such provision, but giving effect, in either case, to any exemptive relief granted to us by the SEC. Recent legislation introduced in the U.S. House of Representatives, if passed, would modify this section of the 1940 Act and increase the amount of debt that BDCs may incur by modifying the asset coverage percentage from 200% to 150%. In addition, recent legislation introduced in the U.S. Senate would modify SBA regulations in a manner that may permit us to incur additional SBA leverage. As a result, we may be able to incur additional indebtedness in the future and therefore your risk of an investment in the Notes may increase.

Risks Related to Our Investments

Our investments in foreign securities may involve significant risks in addition to the risks inherent in U.S. investments.

        Our investments in foreign securities may involve significant risks in addition to the risks inherent in investments in U.S. securities. Our investment strategy contemplates potential investments in debt securities of foreign companies. Investing in foreign companies may expose us to additional risks not typically associated with investing in securities of U.S. companies. These risks include changes in exchange control regulations, political and social instability, expropriation, imposition of foreign taxes, less liquid markets and less available information than is generally the case in the United States, higher transaction costs, less government supervision of exchanges, brokers and issuers, less developed bankruptcy laws, difficulty in enforcing contractual obligations, lack of uniform accounting and auditing standards and greater price volatility.

        Although most of our investments will be U.S. dollar denominated, any investments denominated in a foreign currency will be subject to the risk that the value of a particular currency will change in relation to one or more other currencies. Among the factors that may affect currency values are trade balances, the level of short-term interest rates, differences in relative values of similar assets in different currencies, long-term opportunities for investment and capital appreciation, and political developments.

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USE OF PROCEEDS

        The net proceeds from the sale of the $             million aggregate principal amount of Notes in this offering are $            , and $            if the underwriter's over-allotment option is exercised in full, after deducting the underwriting discount and estimated offering expenses payable by us.

        We intend to use the net proceeds from this offering to repay outstanding debt borrowed under our $287.5 million Credit Facility. However, through re-borrowing of the repaid amounts under our Credit Facility, we intend to use the net proceeds from this offering to make investments in accordance with our investment objective and strategies described in this prospectus supplement and the accompanying prospectus, to make investments in marketable securities and idle funds investments, which may include investments in secured intermediate term bank debt, rated debt securities and other income producing investments, to pay our operating expenses and other cash obligations, and for general corporate purposes. Our ability to achieve our investment objective may be limited to the extent that the net proceeds from an offering, pending full investment, are held in interest bearing deposits or other short-term instruments. See "Risk Factors—Risks Relating to Our Securities—We may be unable to invest a significant portion of the net proceeds from an offering or from exiting an investment or other capital on acceptable terms, which could harm our financial condition and operating results" in the accompanying prospectus.

        At March 25, 2013, we had approximately $141.0 million outstanding under our $287.5 million Credit Facility. Our Credit Facility matures in September 2017, unless extended, and bears interest, at our election, on a per annum basis equal to (i) the applicable LIBOR rate plus 2.50% or (ii) the applicable base rate plus 1.50%. Amounts repaid under our Credit Facility will remain available for future borrowings. As of December 31, 2012, the interest rate on our Credit Facility was 2.71%.

        Affiliates of Raymond James & Associates, Inc., RBC Capital Markets, LLC and BB&T Capital Markets, underwriters in this offering, act as lenders and/or agents under our Credit Facility. As described above, we intend to use net proceeds of this offering to repay the outstanding indebtedness under this Credit Facility, and such affiliates therefore may receive a portion of the proceeds from this offering through the repayment of those borrowings. See "Underwriting—Conflicts of Interest" below.

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CAPITALIZATION

        The following table sets forth our capitalization:

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on an actual basis as of December 31, 2012; and



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on an as-adjusted basis giving effect to the sale of $             million aggregate principal amount of Notes in this offering, less estimated underwriting discounts and offering expenses payable by us.

        This table should be read in conjunction with "Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Audited Financial Statements" in this prospectus supplement.

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RATIOS OF EARNINGS TO FIXED CHARGES

        The following table contains our ratio of earnings to fixed charges for the periods indicated, computed as set forth below. You should read these ratios of earnings to fixed charges in connection with our consolidated financial statements, including the notes to those statements, included in this prospectus supplement.

        For purposes of computing the ratios of earnings to fixed charges, earnings represent net increase in net assets resulting from operations plus (or minus) income tax expense (benefit) including excise tax expense plus fixed charges. Fixed charges include interest and credit facility fees expense and amortization of debt issuance costs.

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SELECTED FINANCIAL DATA

        The selected financial and other data below reflects the consolidated financial condition and the consolidated statement of operations of Main Street and its subsidiaries as of and for the years ended December 31, 2012, 2011, 2010, 2009 and 2008. The selected financial data at December 31, 2012, 2011, 2010, 2009 and 2008 and for the years ended December 31, 2012, 2011, 2010, 2009 and 2008, have been derived from consolidated financial statements that have been audited by Grant Thornton LLP, an independent registered public accounting firm. You should read this selected financial data in conjunction with our "Management's Discussion and Analysis of Financial Condition and Results of Operations," "Senior Securities" and the financial statements and related notes thereto in the accompanying prospectus and "Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Audited Financial Statements" in this prospectus supplement.

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MANAGEMENT'S DISCUSSION AND ANALYSIS OF
FINANCIAL CONDITION AND RESULTS OF OPERATIONS

        The following discussion should be read in conjunction with our consolidated financial statements and notes thereto contained elsewhere in this prospectus supplement.

        Statements we make in the following discussion which express a belief, expectation or intention, as well as those that are not historical fact, are forward-looking statements that are subject to risks, uncertainties and assumptions. Our actual results, performance or achievements, or industry results, could differ materially from those we express in the following discussion as a result of a variety of factors, including the risks and uncertainties we have referred to under the headings "Supplementary Risk Factors" in this prospectus supplement and "Cautionary Statement Concerning Forward-Looking Statements" and "Risk Factors" in the accompanying prospectus.

ORGANIZATION

        Main Street Capital Corporation ("MSCC") was formed on March 9, 2007 for the purpose of (i) acquiring 100% of the equity interests of Main Street Mezzanine Fund, LP ("MSMF") and its general partner, Main Street Mezzanine Management, LLC ("MSMF GP"), (ii) acquiring 100% of the equity interests of Main Street Capital Partners, LLC (the "Investment Manager"), (iii) raising capital in an initial public offering, which was completed in October 2007 (the "IPO"), and (iv) thereafter operating as an internally managed business development company ("BDC") under the Investment Company Act of 1940, as amended (the "1940 Act"). MSMF is licensed as a Small Business Investment Company ("SBIC") by the United States Small Business Administration ("SBA") and the Investment Manager acts as MSMF's manager and investment adviser. Because the Investment Manager, which employs all of the executive officers and other employees of MSCC, is wholly owned by us, we do not pay any external investment advisory fees, but instead we incur the operating costs associated with employing investment and portfolio management professionals through the Investment Manager. The IPO and related transactions discussed above were consummated in October 2007 and are collectively termed the "Formation Transactions."

        On January 7, 2010, MSCC consummated transactions (the "Exchange Offer") to exchange 1,239,695 shares of its common stock for approximately 88% of the total dollar value of the limited partner interests in Main Street Capital II, LP ("MSC II" and, together with MSMF, the "Funds"). Pursuant to the terms of the Exchange Offer, 100% of the membership interests in the general partner of MSC II, Main Street Capital II GP, LLC ("MSC II GP"), were also transferred to MSCC for no consideration. MSC II commenced operations in January 2006, is an investment fund that operates as an SBIC and is also managed by the Investment Manager. During the first quarter of 2012, MSCC exchanged 229,634 shares of its common stock to acquire all of the remaining minority ownership in the total dollar value of the MSC II limited partnership interests, including approximately 5% owned by affiliates of MSCC (the "Final MSC II Exchange"). After the completion of the Final MSC II Exchange, MSCC owns 100% of MSC II. The Exchange Offer and related transactions, including the transfer of the MSC II GP interests and the Final MSC II Exchange, are collectively termed the "Exchange Offer Transactions."

        MSCC has elected to be treated for federal income tax purposes as a regulated investment company ("RIC") under Subchapter M of the Internal Revenue Code of 1986, as amended (the "Code"). As a result, MSCC generally will not pay corporate-level federal income taxes on any net ordinary income or capital gains that it distributes to its stockholders as dividends.

        MSCC has direct and indirect wholly owned subsidiaries that have elected to be taxable entities (the "Taxable Subsidiaries"). The primary purpose of these entities is to hold certain investments that generate "pass through" income for tax purposes. The Taxable Subsidiaries are each taxed at their normal corporate tax rates based on their taxable income.

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        Unless otherwise noted or the context otherwise indicates, the terms "we," "us," "our" and "Main Street" refer to MSCC and its consolidated subsidiaries, which include the Funds and the Taxable Subsidiaries.

OVERVIEW

        We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market ("LMM") companies and debt capital to middle market ("Middle Market") companies. Our portfolio investments are typically made to support management buyouts, recapitalizations, growth financings, refinancings and acquisitions of companies that operate in diverse industry sectors. We seek to partner with entrepreneurs, business owners and management teams and generally provide "one stop" financing alternatives within our LMM portfolio. We invest primarily in secured debt investments, equity investments, warrants and other securities of LMM companies based in the United States and in secured debt investments of Middle Market companies generally headquartered in the United States. Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company. Our LMM companies generally have annual revenues between $10 million and $150 million, and our LMM portfolio investments generally range in size from $5 million to $25 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies, with annual revenues typically between $150 million and $1.5 billion, and our Middle Market investments generally range in size from $3 million to $15 million. Our other portfolio ("Other Portfolio") investments primarily consist of investments which are not consistent with the typical profiles for our LMM and Middle Market portfolio investments, including investments which may be managed by third parties. In our Other Portfolio, we may incur indirect fees and expenses in connection with investments managed by third parties, such as investments in other investment companies or private funds.

        We seek to fill the current financing gap for LMM businesses, which, historically, have had more limited access to financing from commercial banks and other traditional sources. The underserved nature of the LMM creates the opportunity for us to meet the financing needs of LMM companies while also negotiating favorable transaction terms and equity participations. Our ability to invest across a company's capital structure, from senior secured loans to equity securities, allows us to offer portfolio companies a comprehensive suite of financing options, or a "one stop" financing solution. Providing customized, "one stop" financing solutions has become even more relevant to our LMM portfolio companies in the current investing environment. We generally seek to partner directly with entrepreneurs, management teams and business owners in making our investments. Our LMM portfolio debt investments are generally secured by a first lien on the assets of the portfolio company and typically have a term of between five and seven years. We believe that our LMM investment strategy has a lower correlation to the broader debt and equity markets.

        As of December 31, 2012, we had debt and equity investments in 59 LMM portfolio companies with an aggregate fair value of approximately $510.3 million, a total cost basis of approximately $408.0 million, and a weighted average annual effective yield on our LMM debt investments of approximately 14.2%. As of December 31, 2012, approximately 76% of our total LMM portfolio investments at cost were in the form of debt investments and approximately 94% of such debt investments at cost were secured by first priority liens on the assets of our LMM portfolio companies. At December 31, 2012, we had equity ownership in approximately 90% of our LMM portfolio companies and the average fully diluted equity ownership in those portfolio companies was approximately 32%. As of December 31, 2011, we had debt and equity investments in 54 LMM portfolio companies with an aggregate fair value of approximately $415.7 million, a total cost basis of approximately $349.0 million and a weighted average annual effective yield on our LMM debt

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investments of approximately 14.8%. As of December 31, 2011, approximately 74% of our total LMM portfolio investments at cost were in the form of debt investments and approximately 93% of such debt investments at cost were secured by first priority liens on the assets of our LMM portfolio companies. At December 31, 2011, we had equity ownership in approximately 94% of our LMM portfolio companies and the average fully diluted equity ownership in those portfolio companies was approximately 34%. The weighted average annual yields were computed using the effective interest rates for all debt investments as of December 31, 2012 and 2011, including amortization of deferred debt origination fees and accretion of original issue discount but excluding fees payable upon repayment of the debt investments and any debt investments on non-accrual status.

        In addition to our LMM investment strategy, we pursue investments in Middle Market companies. Our Middle Market portfolio investments primarily consist of direct or secondary investments in interest-bearing debt securities in companies that are generally larger in size than the LMM companies included in our LMM portfolio. Our Middle Market portfolio debt investments are generally secured by either a first or second priority lien on the assets of the portfolio company and typically have a term of between three and five years.

        As of December 31, 2012, we had Middle Market portfolio investments in 85 companies collectively totaling approximately $390.0 million in fair value with a total cost basis of approximately $385.5 million. The weighted average annual revenue for the 85 Middle Market portfolio company investments was approximately $513.5 million as of December 31, 2012. As of December 31, 2012, almost all of our Middle Market portfolio investments were in the form of debt investments and approximately 92% of such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average annual effective yield on our Middle Market portfolio debt investments was approximately 8.8% as of December 31, 2012. As of December 31, 2011, we had Middle Market portfolio investments in 57 companies collectively totaling approximately $226.5 million in fair value with a total cost basis of approximately $228.9 million. The weighted average annual revenue for the 57 Middle Market portfolio company investments was approximately $472.6 million as of December 31, 2011. As of December 31, 2011, almost all of our Middle Market portfolio investments were in the form of debt investments and approximately 82% of such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average annual effective yield on our Middle Market portfolio debt investments was approximately 9.5% as of December 31, 2011. The weighted average annual yields were computed using the effective interest rates for all debt investments as of December 31, 2012 and 2011, including amortization of deferred debt origination fees and accretion of original issue discount but excluding fees payable upon repayment of the debt investments.

        As of December 31, 2012, we had Other Portfolio investments in 3 companies collectively totaling approximately $24.1 million in fair value and approximately $23.6 million in cost basis and which comprised 2.6% of our investment portfolio at fair value as of December 31, 2012. As of December 31, 2011, we had Other Portfolio investments in 3 companies collectively totaling approximately $14.1 million in both fair value and cost basis and which comprised 2.1% of our investment portfolio at fair value as of December 31, 2011.

        Our portfolio investments are generally made through MSCC and the Funds. MSCC and the Funds share the same investment strategies and criteria, although they are subject to different regulatory regimes. An investor's return in MSCC will depend, in part, on the Funds' investment returns as MSMF and MSC II are both wholly owned subsidiaries of MSCC.

        The level of new portfolio investment activity will fluctuate from period to period based upon our view of the current economic fundamentals, our ability to identify new investment opportunities that meet our investment criteria, and our ability to consummate the identified opportunities. The level of new investment activity, and associated interest and fee income, will directly impact future investment

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income. In addition, the level of dividends paid by portfolio companies and the portion of our portfolio debt investments on non-accrual status will directly impact future investment income. While we intend to grow our portfolio and our investment income over the long-term, our growth and our operating results may be more limited during depressed economic periods. However, we intend to appropriately manage our cost structure and liquidity position based on applicable economic conditions and our investment outlook. The level of realized gains or losses and unrealized appreciation or depreciation will also fluctuate depending upon portfolio activity and the performance of our individual portfolio companies. The changes in realized gains and losses and unrealized appreciation or depreciation could have a material impact on our operating results.

        MSCC and its consolidated subsidiaries are internally managed by the Investment Manager, a wholly owned subsidiary of MSCC, which employs all of the executive officers and other employees of Main Street. Because the Investment Manager is wholly owned by MSCC, Main Street does not pay any external investment advisory fees, but instead incurs the operating costs associated with employing investment and portfolio management professionals through the Investment Manager. We believe that our internally managed structure provides us with a beneficial operating expense structure when compared to other publicly-traded and privately-held investment firms which are externally managed, and our internally managed structure allows us the opportunity to leverage our non-interest operating expenses as we grow our investment portfolio. For the years ended December 31, 2012 and 2011, the ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets was 1.8% and 2.2% respectively.

        In addition, during May 2012, we and the Investment Manager executed an investment sub-advisory agreement with HMS Adviser, LP, which is the investment advisor to HMS Income Fund, Inc., a non publicly-traded BDC whose registration statement on Form N-2 was declared effective by the SEC in June 2012, to provide certain investment advisory services to HMS Adviser, LP. We are initially providing such investment advisory services to HMS Adviser, LP, but ultimately intend that the Investment Manager provide such services because the fees we receive from such arrangement could otherwise have negative consequences on our ability to meet the source-of-income requirement necessary for us to maintain our RIC tax treatment. We will need to obtain certain relief from the SEC before the Investment Manager is permitted to provide these services to HMS Adviser, LP, which we are seeking, but there can be no assurance that we will obtain such relief.

CRITICAL ACCOUNTING POLICIES

Basis of Presentation

        Our financial statements are prepared in accordance with generally accepted accounting principles in the United States of America ("U.S. GAAP"). For the three years ended December 31, 2012, 2011 and 2010, our consolidated financial statements include the accounts of MSCC and its consolidated subsidiaries, which include the Funds and the Taxable Subsidiaries. Portfolio investments, as used herein, refers to all of our portfolio investments in LMM companies, Middle Market portfolio investments, Other Portfolio investments and our investment in the Investment Manager but excludes all of our "Marketable securities and idle funds investments." Marketable securities and idle funds investments are classified as financial instruments and are reported separately on our Consolidated Balance Sheets and Consolidated Schedule of Investments due to the nature of such investments. Our results of operations for the three years ended December 31, 2012, 2011 and 2010, cash flows for the three years ended December 31, 2012, 2011 and 2010 and financial position as of December 31, 2012 and 2011, are presented on a consolidated basis. The effects of all intercompany transactions between Main Street and its consolidated subsidiaries have been eliminated in consolidation. Certain reclassifications have been made to prior period balances to conform with the current financial statement presentation, including certain investments previously classified as Marketable securities and

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idle funds investments that are now considered a part of the Middle Market portfolio and are now classified as "Non-Control/Non-Affiliate investments."

        Under the investment company rules and regulations pursuant to Article 6 of Regulation S-X and the Audit and Accounting Guide for Investment Companies issued by the American Institute of Certified Public Accountants (the "AICPA Guide"), we are precluded from consolidating portfolio company investments, including those in which we have a controlling interest, unless the portfolio company is another investment company. An exception to this general principle in the AICPA Guide occurs if we own a controlled operating company that provides all or substantially all of its services directly to us, or to an investment company of ours. None of the investments made by us qualify for this exception. Therefore, our portfolio investments are carried on the balance sheet at fair value, as discussed further in Note B to our consolidated financial statements, with any adjustments to fair value recognized as "Net Change in Unrealized Appreciation (Depreciation)" on our Statement of Operations until the investment is realized, usually upon exit, resulting in any gain or loss being recognized as a "Net Realized Gain (Loss) from Investments."

Portfolio Investment Valuation

        The most significant determination inherent in the preparation of our consolidated financial statements is the valuation of our portfolio investments and the related amounts of unrealized appreciation and depreciation. As of December 31, 2012 and 2011, approximately 89% of our total assets at each date represented investments in portfolio companies valued at fair value (including our investment in the Investment Manager). We are required to report our investments at fair value. We follow the provisions of the Financial Accounting Standards Board ("FASB") Accounting Standards Codification ("Codification" or "ASC") 820, Fair Value Measurements and Disclosures ("ASC 820"). ASC 820 defines fair value, establishes a framework for measuring fair value, establishes a fair value hierarchy based on the quality of inputs used to measure fair value, and enhances disclosure requirements for fair value measurements.

        Our business strategy calls for us to invest primarily in illiquid securities issued by private, LMM companies and debt securities issued by Middle Market companies that are generally larger in size than the LMM companies. Our portfolio also includes Other Portfolio investments which primarily consist of investments which are not consistent with the typical profiles for our LMM and Middle Market portfolio investments, including investments which may be managed by third parties. All of our portfolio investments may be subject to restrictions on resale. LMM investments and Other Portfolio investments generally have no established trading market while Middle Market securities generally have established markets that are not active. We determine in good faith the fair value of our portfolio investments pursuant to a valuation policy in accordance with ASC 820 and a valuation process approved by our Board of Directors and in accordance with the 1940 Act. For LMM investments, we review external events, including private mergers, sales and acquisitions involving comparable companies, and include these events in the valuation process. For Middle Market portfolio debt and Other Portfolio debt investments, we primarily use observable inputs such as quoted prices in the valuation process. For Other Portfolio equity investments we generally value such investments based on the fair value of the portfolio company as determined by independent third parties, and based on our proportional ownership in the portfolio company, as well as the financial position and assessed risk of each of these portfolio investments. Our valuation policy and process is intended to provide a consistent basis for determining the fair value of our portfolio.

        For valuation purposes, "control" LMM portfolio investments are composed of debt and equity securities in companies for which we have a controlling interest in the portfolio company or the ability to nominate a majority of the portfolio company's board of directors. Market quotations are generally not readily available for our control LMM portfolio investments. As a result, for control LMM portfolio investments, we determine the fair value using a combination of market and income

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approaches. Under the market approach, we will typically use the enterprise value methodology to determine the fair value of these investments. The enterprise value is the fair value at which an enterprise could be sold in a transaction between two willing parties, other than through a forced or liquidation sale. Typically, private companies are bought and sold based on multiples of earnings before interest, taxes, depreciation and amortization, or EBITDA, cash flows, net income, revenues, or in limited cases, book value. There is no single methodology for estimating enterprise value. For any one portfolio company, enterprise value is generally described as a range of values from which a single estimate of enterprise value is derived. In estimating the enterprise value of a portfolio company, we analyze various factors, including the portfolio company's historical and projected financial results. We allocate the enterprise value to investments in order of the legal priority of the various components of the portfolio company's capital structure. We will also use the income approach to determine the fair value of these securities, based on projections of the discounted future free cash flows that the portfolio company or the debt security will likely generate, and which includes using a yield-to-maturity approach that analyzes the discounted cash flows of interest and principal for the debt security, as set forth in the associated loan agreements, as well as the financial position and credit risk of each of these portfolio investments. The valuation approaches for our control LMM portfolio investments estimate the value of the investment if we were to sell, or exit, the investment. In addition, these valuation approaches consider the value associated with our ability to control the capital structure of the portfolio company, as well as the timing of a potential exit.

        For valuation purposes, "non-control" LMM portfolio investments are composed of debt and equity securities in companies for which we do not have a controlling interest in the portfolio company or the ability to nominate a majority of the portfolio company's board of directors. Market quotations are generally not readily available for non-control LMM portfolio investments. For our non-control LMM investments, we use a combination of the market and income approaches to value our equity investments and the income approach to value our debt investments similar to the approaches used for our control LMM portfolio investments, and which includes using a yield-to-maturity approach that analyzes the discounted cash flows of interest and principal for the debt security, as set forth in the associated loan agreements, as well as the financial position and credit risk of each of these portfolio investments. Our estimate of the expected repayment date of a LMM debt security is generally the legal maturity date of the instrument, as we generally intend to hold our LMM loans and debt securities to maturity. The yield-to-maturity analysis considers changes in leverage levels, credit quality, portfolio company performance and other factors. We will use the value determined by the yield-to-maturity analysis as the fair value for that security; however, because of our general intent to hold our loans to maturity, the fair value will not exceed the face amount of the LMM debt security. A change in the assumptions that we use to estimate the fair value of our LMM debt securities using the yield-to-maturity analysis could have a material impact on the determination of fair value. If there is deterioration in credit quality or if a LMM debt security is in workout status, we may consider other factors in determining the fair value of the LMM debt security, including the value attributable to the debt security from the enterprise value of the portfolio company or the proceeds that would most likely be received in a liquidation analysis.

        Our Middle Market portfolio investments primarily consist of direct or secondary investments in interest-bearing debt securities in companies that are generally larger in size than the LMM companies included in our investment portfolio. For valuation purposes, all of our Middle Market portfolio investments are non control investments for which we do not have a controlling interest in the portfolio company, or the ability to nominate a majority of the portfolio company's board of directors. We primarily use observable inputs to determine the fair value of these investments through obtaining third party quotes or other independent pricing. For Middle Market portfolio investments for which sufficient observable inputs are not available to determine fair value, we use a combination of observable inputs through obtaining third party quotes or other independent pricing and an approach

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that is similar to the income approach using a yield to maturity model used to value our LMM portfolio debt investments.

        For valuation purposes, all of our Other Portfolio investments are non-control investments for which we generally do not have a controlling interest in the portfolio company or the ability to nominate a majority of the portfolio company's board of directors. Similar to the LMM investment portfolio, market quotations for Other Portfolio equity investments are generally not readily available. We value our Other Portfolio equity investments based on the fair value of the portfolio company as determined by independent third parties and based on our proportional ownership in the portfolio company, as well as the financial position and assessed risk of each of these portfolio investments. For Other Portfolio debt investments with observable inputs, we determine the fair value of these investments through obtaining third party quotes or other independent pricing. To the extent observable inputs are not available for our Other Portfolio debt investments, we value these Other Portfolio debt investments through an approach similar to the income approach using a yield-to-maturity model used to value our non-control LMM portfolio debt investments.

        Due to the inherent uncertainty in the valuation process, our determination of fair value for our portfolio investments may differ materially from the values that would have been used had a ready market for the securities existed. In addition, changes in the market environment, portfolio company performance and other events that may occur over the lives of the investments may cause the gains or losses ultimately realized on these investments to be materially different than the valuations currently assigned. We determine the fair value of each individual investment and record changes in fair value as unrealized appreciation or depreciation.

Revenue Recognition

Interest and Dividend Income

        We record interest and dividend income on the accrual basis to the extent amounts are expected to be collected. Dividend income is recorded as dividends are declared or at the point an obligation exists for the portfolio company to make a distribution. In accordance with our valuation policy, we evaluate accrued interest and dividend income periodically for collectability. When a loan or debt security becomes 90 days or more past due, and if we otherwise do not expect the debtor to be able to service all of its debt or other obligations, we will generally place the loan or debt security on non-accrual status and cease recognizing interest income on that loan or debt security until the borrower has demonstrated the ability and intent to pay contractual amounts due. If a loan or debt security's status significantly improves regarding the debtor's ability to service the debt or other obligations, or if a loan or debt security is fully impaired, sold or written off, we will remove it from non-accrual status.

Fee Income

        We may periodically provide services, including structuring and advisory services, to our portfolio companies. For services that are separately identifiable and evidence exists to substantiate fair value, income is recognized as earned, which is generally when the investment or other applicable transaction closes. Fees received in connection with debt financing transactions for services that do not meet these criteria are treated as debt origination fees and are deferred and accreted into interest income over the life of the financing.

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Payment-in-Kind ("PIK") Interest and Cumulative Dividends

        We hold debt and preferred equity instruments in our investment portfolio that contain payment-in-kind ("PIK") interest and cumulative dividend provisions. The PIK interest, computed at the contractual rate specified in each debt agreement, is periodically added to the principal balance of the debt and is recorded as interest income. Thus, the actual collection of this interest may be deferred until the time of debt principal repayment. Cumulative dividends are recorded as dividend income, and any unpaid dividends are added to the balance of the preferred equity investment. The actual collection of these dividends may be deferred until such time as the preferred equity is redeemed. To maintain RIC tax treatment (as discussed below), these non-cash sources of income may need to be paid out to stockholders in the form of distributions, even though we may not have collected the PIK interest and cumulative dividends in cash. We will stop accruing PIK interest and cumulative dividends and will write off any accrued and uncollected interest and dividends in arrears when it is determined that such PIK interest and dividends in arrears are no longer collectible.

Share-Based Compensation

        We account for our share-based compensation plans using the fair value method, as prescribed by ASC 718, Compensation—Stock Compensation. Accordingly, for restricted stock awards, we measured the grant date fair value based upon the market price of our common stock on the date of the grant and will amortize this fair value to share-based compensation expense over the requisite service period or vesting term.

Income Taxes

        MSCC has elected to be treated for federal income tax purposes as a RIC. As a RIC, MSCC generally will not pay corporate-level federal income taxes on any net ordinary income or capital gains that MSCC distributes to its stockholders as dividends. MSCC must generally distribute at least 90% of its investment company taxable income to qualify for pass-through tax treatment and maintain its RIC status. As part of maintaining RIC status, undistributed taxable income (subject to a 4% excise tax) pertaining to a given fiscal year may be distributed up to 12 months subsequent to the end of that fiscal year, provided such dividends are declared prior to the filing of the federal income tax return for the prior year.

        The Taxable Subsidiaries hold certain portfolio investments for us. The Taxable Subsidiaries are consolidated with us for financial reporting purposes, and the portfolio investments held by the Taxable Subsidiaries are included in our consolidated financial statements. The principal purpose of the Taxable Subsidiaries is to permit us to hold equity investments in portfolio companies which are "pass through" entities for tax purposes in order to comply with the "source income" requirements contained in the RIC tax provisions of the Code. The Taxable Subsidiaries are not consolidated with us for income tax purposes and may generate income tax expense or income tax benefit as a result of their ownership of various portfolio investments. This income tax expense or benefit, if any, is reflected in our Consolidated Statement of Operations.

        The Taxable Subsidiaries use the liability method in accounting for income taxes. Deferred tax assets and liabilities are recorded for temporary differences between the tax basis of assets and liabilities and their reported amounts in the financial statements, using statutory tax rates in effect for the year in which the temporary differences are expected to reverse. A valuation allowance is provided against deferred tax assets when it is more likely than not that some portion or all of the deferred tax asset will not be realized.

        Taxable income generally differs from net income for financial reporting purposes due to temporary and permanent differences in the recognition of income and expenses. Taxable income

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generally excludes net unrealized appreciation or depreciation, as investment gains or losses are not included in taxable income until they are realized.

PORTFOLIO INVESTMENT COMPOSITION

        LMM portfolio investments principally consist of secured debt, equity warrants and direct equity investments in privately held, LMM companies. The LMM debt investments are primarily secured by either a first or second lien on the assets of the portfolio company, generally bear interest at fixed rates, and generally mature between five and seven years from the original investment date. In most LMM portfolio companies, we also receive nominally priced equity warrants and/or make direct equity investments, usually in connection with a debt investment.

        Middle Market portfolio investments primarily consist of direct or secondary investments in interest-bearing debt securities in companies that are generally larger in size than the LMM companies included in our LMM portfolio. Our Middle Market portfolio debt investments are generally secured by either a first or second priority lien.

        The following table summarizes the composition of our LMM investment portfolio, Middle Market investment portfolio, and total combined LMM and Middle Market investment portfolio at cost and fair value by type of investment as a percentage of the total LMM investment portfolio, the total Middle Market investment portfolio and the total combined LMM and Middle Market investment portfolio as of December 31, 2012 and 2011 (this information excludes the Other Portfolio investments and the Investment Manager).

 

        The following table shows the LMM investment portfolio, the Middle Market investment portfolio, and the total combined LMM and Middle Market investment portfolio composition by geographic region of the United States or other countries at cost and fair value as a percentage of the total LMM investment portfolio, the total Middle Market investment portfolio, and the total combined LMM and Middle Market investment portfolio, as of December 31, 2012 and 2011 (this information excludes the

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Other Portfolio investments and the Investment Manager). The geographic composition is determined by the location of the corporate headquarters of the portfolio company.

 

        Our LMM and Middle Market portfolio investments are in companies conducting business in a variety of industries. The following tables show the composition of our LMM portfolio investments, Middle Market portfolio investments, and total combined LMM and Middle Market portfolio

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investments, by industry at cost and fair value as of December 31, 2012 and 2011 (this information excludes the Other Portfolio investments and the Investment Manager).

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        Our LMM, Middle Market and Other Portfolio investments carry a number of risks including, but not limited to: (1) investing in LMM, Middle Market and Other Portfolio companies which may have limited operating histories and financial resources; (2) holding investments that generally are not publicly traded and which may be subject to legal and other restrictions on resale; and (3) other risks common to investing in below investment grade debt and equity investments. Please see "Risk Factors—Risks Related to Our Investments" for a more complete discussion of the risks involved with investing in our portfolio companies.

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PORTFOLIO ASSET QUALITY

        We utilize an internally developed investment rating system to rate the performance of each LMM portfolio company and to monitor our expected level of returns on each of our LMM investments in relation to our expectations for the portfolio company. The investment rating system takes into consideration various factors, including but not limited to each investment's expected level of returns and the collectability of our debt investments, comparisons to competitors and other industry participants and the portfolio company's future outlook.

•

Investment Rating 1 represents a LMM portfolio company that is performing in a manner which significantly exceeds expectations.



•

Investment Rating 2 represents a LMM portfolio company that, in general, is performing above expectations.



•

Investment Rating 3 represents a LMM portfolio company that is generally performing in accordance with expectations.



•

Investment Rating 4 represents a LMM portfolio company that is underperforming expectations. Investments with such a rating require increased monitoring and scrutiny by us.



•

Investment Rating 5 represents a LMM portfolio company that is significantly underperforming. Investments with such a rating require heightened levels of monitoring and scrutiny by us and involve the recognition of significant unrealized depreciation on such investment.

        All new LMM portfolio investments receive an initial Investment Rating of 3.

        The following table shows the distribution of our LMM portfolio investments on the 1 to 5 investment rating scale at fair value as of December 31, 2012 and 2011.

        Based upon our investment rating system, the weighted average rating of our LMM portfolio was approximately 2.1 as of December 31, 2012 and 2.2 as of December 31, 2011.

        For the total investment portfolio, as of December 31, 2012, we had no investments with positive fair value on non-accrual status and one fully impaired investment which comprised approximately 0.2% of the total portfolio investments at cost, excluding the investment in the affiliated Investment Manager. As of December 31, 2011, we had one investment with positive fair value on non-accrual status, which comprised less than 0.1% of the total portfolio investments at fair value and, together with another fully impaired investment, comprised approximately 0.9% of the total portfolio investments at cost, in each case excluding the investment in the affiliated Investment Manager.

        The broader fundamentals of the United States economy remain mixed, and unemployment remains elevated. In the event that the United States economy contracts, it is likely that the financial results of small- to mid-sized companies, like those in which we invest, could experience deterioration or limited growth from current levels, which could ultimately lead to difficulty in meeting their debt

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service requirements and an increase in defaults. Consequently, we can provide no assurance that the performance of certain portfolio companies will not be negatively impacted by economic cycles or other conditions, which could also have a negative impact on our future results.

DISCUSSION AND ANALYSIS OF RESULTS OF OPERATIONS

Comparison of years ended December 31, 2012 and December 31, 2011

 

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Investment Income

        For the year ended December 31, 2012, total investment income was $90.5 million, a $24.3 million, or 37%, increase over the $66.2 million for the corresponding period of 2011. This comparable period increase was principally attributable to (i) a $19.1 million increase in interest income from increased activity in the investment portfolio and higher average levels of portfolio debt investments and interest-bearing marketable securities investments, (ii) a $3.2 million increase in dividend income from portfolio equity investments and (iii) a $2.0 million increase in fee income due to the increased activity in and size of the investment portfolio. The increase in investment income included (i) $1.8 million of non-recurring investment income during the first quarter of 2012 associated with repayment and financing activities for two LMM portfolio investments, (ii) a $3.2 million increase in investment income associated with higher levels of accelerated prepayment activity for certain Middle Market portfolio debt investments and marketable securities investments in comparison to 2011 and (iii) special dividend activity of $1.4 million in the fourth quarter of 2012.

Expenses

        For the year ended December 31, 2012, total expenses increased by approximately $4.3 million, or 16%, to $31.2 million from $26.9 million for the corresponding period of 2011. This comparable period increase in expenses was principally attributable to (i) higher interest expense of $2.1 million as a result of the net issuance of an additional $5 million in SBIC debentures subsequent to December 31, 2011, increased borrowing activity under the Credit Facility and higher unused fees associated with the increased commitments under the Credit Facility, (ii) higher share-based compensation expense of $0.5 million related to non-cash amortization for restricted share grants, and (iii) higher compensation and expenses of $1.7 million related to increases in personnel and incentive compensation compared to the corresponding period of 2011. For the years ended December 31, 2012 and 2011, the ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets was 1.8% and 2.2%, respectively.

Distributable Net Investment Income

        Distributable net investment income for the year ended December 31, 2012 increased to $61.9 million, or $2.09 per share, compared with distributable net investment income of $41.3 million, or $1.77 per share, for the corresponding period of 2011. The increase in distributable net investment income was primarily due to the higher level of total investment income partially offset by higher interest and other operating expenses, due to the changes discussed above. Distributable net investment income on a per share basis for the year ended 2012 reflects (i) an increase of approximately $0.13 per share from 2011 in investment income attributable to higher levels of accelerated prepayment and repricing activity for certain debt investments and marketable securities investments, (ii) approximately $0.05 per share from the special dividend activity in the fourth quarter of 2012 and (iii) a greater number of average shares outstanding compared to the corresponding period in 2011 primarily due to the net effect of December 2012, June 2012, October 2011 and March 2011 follow-on stock offerings.

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Net Investment Income

        Net investment income for the year ended December 31, 2012 was $59.3 million, or a 51% increase, compared to net investment income of $39.3 million for the corresponding period of 2011. The increase in net investment income was principally attributable to the increase in total investment income partially offset by higher interest and other operating expenses as discussed above.

Distributable Net Realized Income

        Distributable net realized income increased to $78.4 million, or $2.65 per share, for the year ended 2012 compared with distributable net realized income of $44.0 million, or $1.89 per share, for the corresponding period of 2011. The increase was primarily attributable to the higher level of distributable net investment income and the higher level of total net realized gain from investments in 2012 compared to the corresponding period of 2011. The $16.5 million net realized gain during 2012 was primarily attributable to (i) realized gains recognized on two partial exits of LMM portfolio company equity investments, (ii) a realized gain recognized on the full exit of a LMM portfolio company equity investment and (iii) realized gains related to Middle Market and marketable securities investments, partially offset by (iv) realized losses on the full exits of three LMM portfolio company investments.

Net Realized Income

        The higher level of net investment income and the higher level of total net realized gain from investments in 2012 compared to the corresponding period of 2011, both as discussed above, resulted in a $33.8 million increase in net realized income compared with the corresponding period of 2011.

Net Increase in Net Assets Resulting from Operations

        The net increase in net assets resulting from operations attributable to common stock during the year ended December 31, 2012 was $104.4 million, or $3.53 per share, compared with a net increase of $63.0 million, or $2.76 per share, in 2011. This $41.4 million increase was a result of the increase in net realized income discussed above, plus differences in the net change in unrealized appreciation from portfolio investments, marketable securities, SBIC debentures and investment in the Investment Manager and the difference in the income tax provision. For the year ended December 31, 2012, the $44.5 million net change in unrealized appreciation from portfolio investments was principally attributable to (i) unrealized appreciation on 37 LMM portfolio investments totaling $57.8 million, partially offset by unrealized depreciation on 10 LMM portfolio investments totaling $4.6 million, (ii) $9.7 million of net unrealized appreciation on the Middle Market investment portfolio and (iii) $0.8 million of net unrealized appreciation on the Other Portfolio investments and Marketable securities and idle funds investments, partially offset by (iv) accounting reversals of net unrealized appreciation from prior periods of $18.3 million related to portfolio investment exits and repayments, and (v) accounting reversals of net unrealized appreciation from prior periods of $0.5 million related to Marketable securities and idle funds investments exits and repayments. For the year ended December 31, 2012, the $5.0 million net change in unrealized appreciation attributable to SBIC debentures and investment in the Investment Manager was primarily attributable to unrealized depreciation on the SBIC debentures held by MSC II. The noncontrolling interest of $0.1 million recognized during the first quarter of 2012 reflects the pro rata portion of the net increase in net assets resulting from operations for MSC II attributable to the equity interests in MSC II that were not owned by MSCC prior to MSCC's completion of the Final MSC II Exchange. For the year ended December 31, 2012, we also recognized a net income tax provision of $10.8 million related to deferred taxes of $8.0 million and other taxes of $2.8 million. The deferred taxes related primarily to net unrealized appreciation on equity investments held in our taxable subsidiaries. The other taxes include

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$1.6 million related to an accrual for excise tax on our estimated spillover taxable income as of December 31, 2012 and $1.2 million related to accruals for state and other taxes.

Comparison of years ended December 31, 2011 and December 31, 2010

 

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Investment Income

        For the year ended December 31, 2011, total investment income was $66.2 million, a $29.7 million, or 81%, increase over the $36.5 million of total investment income for the corresponding period of 2010. This comparable period increase was principally attributable to (i) a $23.8 million increase in interest income from higher average levels of both portfolio debt investments and interest-bearing marketable securities investments, (ii) a $4.3 million increase in dividend income from portfolio equity investments, and (iii) a $1.6 million increase in fee income due to higher levels of transaction activity. The increase in investment income included a $2.7 million increase in investment income associated with higher levels of accelerated prepayment and repricing activity for certain debt investments.

Expenses

        For the year ended December 31, 2011, total expenses increased by approximately $9.7 million, or 56%, to $26.9 million from $17.2 million for the corresponding period of 2010. This comparable period increase in expenses was principally attributable to (i) higher interest expense of $4.5 million as a result of the issuance of an additional $40 million in SBIC debentures subsequent to December 31, 2010, and increased borrowing activity under the Credit Facility, (ii) higher share-based compensation expense of $0.6 million related to non-cash amortization for restricted share grants, and (iii) higher compensation and other operating expenses of $4.7 million related to the significant increase in investment income and portfolio investments compared to the corresponding period of 2010. The ratio of total operating expenses, excluding interest expense, as a percentage of average total assets for the year ended December 31, 2011 was 2.2%, representing an approximate 7% decrease from the same ratio of 2.4% for the year ended December 31, 2010.

Distributable Net Investment Income

        Distributable net investment income for the year ended December 31, 2011 increased to $41.3 million, or $1.77 per share, compared with distributable net investment income of $20.7 million, or $1.25 per share, for the corresponding period of 2010. The increase in distributable net investment income was primarily due to the higher level of total investment income partially offset by higher interest and other operating expenses, due to the changes discussed above. Distributable net investment income on a per share basis for the year ended 2011 reflects approximately $0.12 per share of investment income associated with higher levels of accelerated prepayment and repricing activity for certain debt investments and (ii) a greater number of average shares outstanding compared to the corresponding period in 2010 primarily due to the October 2011, March 2011, and August 2010 follow-on stock offerings.

Net Investment Income

        Net investment income for the year ended December 31, 2011 was $39.3 million, or a 104% increase, compared to net investment income of $19.3 million for the corresponding period of 2010. The increase in net investment income was principally attributable to the increase in total investment income partially offset by higher interest and other operating expenses as discussed above.

Distributable Net Realized Income

        Distributable net realized income increased to $44.0 million, or $1.89 per share, for the year ended 2011 compared with distributable net realized income of $17.8 million, or $1.08 per share, for the corresponding period of 2010. The increase was primarily attributable to the higher level of distributable net investment income as well as the higher level of total net realized gain from investments in 2011 compared to the net realized loss from investments in the corresponding period of 2010. The $2.6 million net realized gain during 2011 was primarily attributable to (i) realized gain

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recognized on one partial exit of an LMM portfolio company equity investment, (ii) realized gain recognized on one full exit of an LMM portfolio company equity investment, and (iii) realized gains related to Middle Market and marketable securities investments. The $2.9 million net realized loss during the 2010 year was primarily attributable to $5.9 million of realized loss from our debt and equity investments in two portfolio companies, partially offset by (i) $2.3 million of realized gain on two partial exits and one full exit of portfolio company equity investments and (ii) $0.7 million of realized gain related to Middle Market and marketable securities and idle funds investments.

Net Realized Income

        The higher level of net investment income and the change from net realized loss to net realized gain from investments during 2011 resulted in a $25.6 million increase in net realized income compared with the corresponding period of 2010.

Net Increase in Net Assets Resulting from Operations

        For the year ended December 31, 2011, the $28.4 million net change in unrealized appreciation was principally attributable to (i) unrealized appreciation on 30 LMM portfolio investments totaling $53.6 million, partially offset by unrealized depreciation on 11 LMM portfolio investments totaling $11.8 million, (ii) $3.7 million of net unrealized depreciation on Middle Market portfolio investments and marketable securities and idle funds investments, (iii) accounting reversals of net unrealized appreciation related to the net realized gains recognized during 2011 in the amounts of $2.8 million for portfolio investments and $0.4 million for marketable securities and idle funds investments, (iv) $6.3 million of net unrealized depreciation attributable to our SBIC debentures, and (v) $0.2 million in unrealized depreciation attributable to our investment in the affiliated Investment Manager. The noncontrolling interest of $1.1 million recognized during 2011 reflects the pro rata portion of MSC II net earnings attributable to the equity interests in MSC II not owned by Main Street. For the year ended December 31, 2011, we also recognized a net income tax provision of $6.3 million principally related to deferred taxes on net unrealized appreciation of certain portfolio investments held in our Taxable Subsidiaries.

        As a result of these events, our net increase in net assets resulting from operations attributable to common stock during 2011 was $63.0 million, or $2.76 per share, compared with a net increase in net assets resulting from operations attributable to common stock of $38.7 million, or $2.38 per share, in 2010.

Liquidity and Capital Resources

Cash Flows

        For the year ended December 31, 2012, we experienced a net increase in cash and cash equivalents in the amount of $20.9 million. During that period, we generated $48.9 million of cash from our operating activities, primarily from (i) distributable net investment income, excluding the non-cash effects of the accretion of unearned income, payment-in-kind interest income and the amortization of deferred financing costs, (ii) increases in payables, and (iii) realized gains, partially offset by increases in interest receivable. We used $184.5 million in net cash from investing activities, principally including the funding of $639.8 million for new portfolio company investments and the funding of $14.4 million for Marketable securities and idle funds investments, partially offset by (i) $400.0 million in cash proceeds from the repayment of portfolio debt investments, (ii) $35.1 million in cash proceeds from the exit of portfolio equity investments and (iii) $34.5 million of cash proceeds from the sale of Marketable securities and idle funds investments. During 2012, $156.5 million in cash was provided by financing activities, which principally consisted of (i) $169.9 million in net cash proceeds from public stock offerings in June and December 2012, (ii) $25.0 million in net cash proceeds from the Credit Facility and (iii) $5.0 million in net cash proceeds from the issuance of SBIC debentures, partially offset by (i) $39.9 million in cash dividends paid to stockholders and (ii) $2.2 million in loan costs associated with our SBIC debentures and the Credit Facility.

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        For the year ended December 31, 2011, we experienced a net increase in cash and cash equivalents in the amount of $20.3 million. During that period, we generated $37.2 million of cash from our operating activities, primarily from (i) distributable net investment income, excluding the non-cash effects of the accretion of unearned income, payment-in-kind interest income and the amortization of deferred financing costs, (ii) increases in payables, and (iii) realized gains, partially offset by (iv) increases in interest receivable. We used $220.5 million in net cash from investing activities, principally including (i) the funding of $358.9 million for new portfolio company investments and (ii) the funding of $33.5 million for Marketable securities and idle funds investments, partially offset by (i) $160.2 million in cash proceeds from the repayment of portfolio debt investments and from the exit of portfolio equity investments and (ii) $11.7 million of cash proceeds from the sale of Marketable securities and idle funds investments. During 2011, $203.6 million in cash was provided by financing activities, which principally consisted of (i) $127.8 million in net cash proceeds from public stock offerings in March 2011 and October 2011, (ii) $40.0 million in cash proceeds from the issuance of SBIC debentures, and (iii) $68.0 million in net cash proceeds from the Credit Facility, partially offset by $28.3 million in cash dividends paid to stockholders and $2.3 million in loan costs associated with our SBIC debentures and credit facility.

        For the year ended December 31, 2010, we experienced a net decrease in cash and cash equivalents in the amount of $8.3 million. During that period, we generated $16.6 million of cash from our operating activities, primarily from distributable net investment income partially offset by increases in interest receivable. We used $176.0 million in net cash from investing activities, principally including the funding of $157.7 million for new portfolio company investments and the funding of $100.6 million for Marketable securities and idle funds investments, partially offset by (i) $36.8 million of cash proceeds from the sale of Marketable securities and idle funds investments, (ii) $43.0 million in cash proceeds from the repayment of portfolio debt investments and from the exit of portfolio equity investments, and (iii) $2.5 million in cash acquired as part of the Exchange Offer. During 2010, $151.1 million in cash was provided by financing activities, which principally consisted of (i) $85.9 million in net cash proceeds from public stock offerings in January 2010 and August 2010, (ii) $45.0 million in cash proceeds from the issuance of SBIC debentures, and (iii) $39 million in net cash proceeds from the Credit Facility, partially offset by $16.3 million in cash dividends paid to stockholders and $2.1 million in loan costs associated with our SBIC debentures and the Credit Facility.

Capital Resources

        As of December 31, 2012, we had $63.5 million in cash and cash equivalents and $28.5 million in Marketable securities and idle funds investments. As of December 31, 2012, our net asset value totaled $643.0 million, or $18.59 per share.

        In November 2011, we expanded the Credit Facility from $155 million to $210 million to provide additional liquidity in support of future investment and operational activities. The $55 million increase in total commitments included commitment increases by lenders currently participating in the Credit Facility, as well as the addition of one new lender relationship which diversified our lending group to a total of seven participants. In December 2011, we further expanded the Credit Facility from $210 million to $235 million. The $25 million increase in total commitments included the addition of one new lender relationship which further diversified our lending group to a total of eight participants. In May 2012, we expanded the Credit Facility from $235.0 million to $277.5 million. The $42.5 million increase in total commitments included commitment increases by three lenders currently participating in the Credit Facility. The amended Credit Facility contained an upsized accordion feature that allows for a further increase in total commitments under the facility up to $350.0 million of total commitments from new and existing lenders on the same terms and conditions as the existing commitments. In July 2012, we further expanded the Credit Facility from $277.5 million to $287.5 million. The expansion of the Credit Facility included the addition of one new lender relationship which further diversified the

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Main Street lending group to a total of nine participants. These increases in total commitments were executed under the accordion feature of the Credit Facility which allowed us to increase the total commitments under the facility up to $350 million of total commitments from new or existing lenders on the same terms and conditions as the existing commitments. During November 2012, we amended the Credit Facility to extend the final maturity to five years, through September 2017. The amended Credit Facility contains an upsized accordion feature which allows us to increase the total commitments under the facility up to $400 million from new or existing lenders on the same terms and conditions as the existing commitments. The Credit Facility currently includes an initial revolving period through September 2015 followed by a two-year term out period with a final maturity in September 2017, and contains two, one-year extension options which could extend both the revolving period and the final maturity by up to two years, subject to certain conditions including lender approval. Borrowings under the Credit Facility bear interest, subject to our election, on a per annum basis equal to (i) the applicable LIBOR rate (0.21% as of December 31, 2012) plus 2.50% or (ii) the applicable base rate (Prime Rate, 3.25% as of December 31, 2012) plus 1.50%. We pay unused commitment fees of 0.375% per annum on the average unused lender commitments under the Credit Facility. The Credit Facility is secured by a first lien on the assets of MSCC and its subsidiaries, excluding the assets of the Funds. The Credit Facility contains certain affirmative and negative covenants, including but not limited to: (i) maintaining an interest coverage ratio of at least 2.0 to 1.0, (ii) maintaining an asset coverage ratio of at least 2.5 to 1.0, and (iii) maintaining a minimum tangible net worth. At December 31, 2012, we had $132 million in borrowings outstanding under the Credit Facility, bearing interest at an interest rate of 2.71%. As of December 31, 2012, we were in compliance with all financial covenants of the Credit Facility.

        In March 2011, we completed a follow-on public stock offering in which we sold 4,025,000 shares of common stock, including the underwriters' full exercise of the over-allotment option, at a price to the public of $18.35 per share (or approximately 141% of the then latest reported Net Asset Value per share), resulting in total net proceeds of approximately $70.3 million, after deducting underwriters' commissions and offering costs. In October 2011, we completed a follow-on public stock offering in which we sold 3,450,000 shares of common stock, including the underwriters' full exercise of the over-allotment option, at a price to the public of $17.50 per share (or approximately 123% of the then latest reported Net Asset Value per share), resulting in total net proceeds of approximately $57.5 million, after deducting underwriters' commissions and offering costs. In June 2012, we completed a follow-on public stock offering in which we sold 4,312,500 shares of common stock, including the underwriters' full exercise of the over-allotment option, at a price to the public of $22.50 per share (or approximately 143% of the then latest reported Net Asset Value per share), resulting in total net proceeds of approximately $93.0 million, after deducting underwriters' commissions and offering costs. In December 2012, we completed a follow-on public stock offering in which we sold 2,875,000 shares of common stock, including the underwriters' full exercise of the over-allotment option, at a price to the public of $28.00 per share (or approximately 160% of the then latest reported Net Asset Value per share), resulting in total net proceeds of approximately $77.1 million, after deducting underwriters' commissions and offering costs.

        Due to each of the Funds' status as a licensed SBIC, we have the ability to issue, through the Funds, debentures guaranteed by the SBA at favorable interest rates. Under the regulations applicable to SBIC funds, an SBIC can have outstanding debentures guaranteed by the SBA generally in an amount up to twice its regulatory capital, which effectively approximates the amount of its equity capital. Debentures guaranteed by the SBA have fixed interest rates that equal prevailing 10-year Treasury Note rates plus a market spread and have a maturity of ten years with interest payable semi-annually. The principal amount of the debentures is not required to be paid before maturity but may be pre-paid at any time with no prepayment penalty. On December 31, 2012, we, through the Funds, had $225 million of outstanding indebtedness guaranteed by the SBA, which carried a weighted average annual fixed interest rate of approximately 4.7%. The first maturity related to the SBIC

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debentures does not occur until 2014, and the remaining weighted average duration is approximately 6.4 years as of December 31, 2012. During the year ended December 31, 2012, we voluntarily prepaid $16 million of SBIC debentures and issued $21 million of new SBIC debentures.

        We anticipate that we will continue to fund our investment activities through existing cash and cash equivalents, the liquidation of Marketable securities and idle funds investments, and a combination of future debt and equity capital. Our primary uses of funds will be investments in portfolio companies, operating expenses and cash distributions to holders of our common stock.

        We periodically invest excess cash balances into Marketable securities and idle funds investments. The primary investment objective of Marketable securities and idle funds investments is to generate incremental cash returns on excess cash balances prior to utilizing those funds for investment in our LMM and Middle Market portfolio investment strategy. Marketable securities and idle funds investments generally consist of debt investments, independently rated debt investments, certificates of deposit with financial institutions, and diversified bond funds. The composition of Marketable securities and idle funds investments will vary in a given period based upon, among other things, changes in market conditions, the underlying fundamentals in our Marketable securities and idle funds investments, our outlook regarding future LMM and Middle Market portfolio investment needs, and any regulatory requirements applicable to Main Street.

        If our common stock trades below our net asset value per share, we will generally not be able to issue additional common stock at the market price unless our stockholders approve such a sale and our Board of Directors makes certain determinations. A proposal, approved by our stockholders at our June 2012 annual meeting of stockholders, authorizes us to sell shares of our common stock below the then current net asset value per share of our common stock in one or more offerings for the period ending on the earlier of (i) June 14, 2013, the one year anniversary of our 2012 annual meeting of stockholders, or (ii) the date of our 2013 annual meeting of stockholders. We would need similar future approval from our stockholders to issue shares below the then current net asset value per share any time after the expiration of the current approval. We do not currently expect to seek such approval at our 2013 annual meeting of stockholders because our common stock price per share has been trading significantly above the current net asset value per share of our common stock.

        In order to satisfy the Code requirements applicable to a RIC, we intend to distribute to our stockholders substantially all of our taxable income, but we may also elect to periodically spillover certain excess undistributed taxable income from one tax year into the next tax year. In addition, as a BDC, we generally are required to meet a coverage ratio of total assets to total senior securities, which include borrowings and any preferred stock we may issue in the future, of at least 200%. This requirement limits the amount that we may borrow. In January 2008, we received an exemptive order from the SEC to exclude SBA-guaranteed debt securities issued by MSMF and any other wholly owned subsidiaries of ours which operate as SBICs from the asset coverage requirements of the 1940 Act as applicable to Main Street, which, in turn, enables us to fund more investments with debt capital.

        Although we have been able to secure access to additional liquidity, including recent public stock offerings, our expanded $287.5 million Credit Facility, and the available leverage through the SBIC program, there is no assurance that debt or equity capital will be available to us in the future on favorable terms, or at all.

Recently Issued Accounting Standards

        In May 2011, the FASB issued Accounting Standards Update ("ASU") 2011-04, Fair Value Measurements (Topic 820), Amendments to Achieve Common Fair Value Measurement and Disclosure Requirements in U.S. GAAP and IFRSs ("ASU 2011-04"). ASU 2011-04 results in common fair value measurement and disclosure requirements in U.S. GAAP and IFRSs. ASU 2011-04 is effective for

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interim and annual reporting periods beginning after December 15, 2011. The adoption of ASU 2011-04 did not have a significant impact on Main Street's financial condition and results of operations.

        In February 2011, the FASB issued ASU 2011-02, Receivables (Topic 310): A Creditor's Determination of Whether a Restructuring is a Troubled Debt Restructuring ("ASU 2011-02"). ASU 2011-02 clarifies which loan modifications constitute troubled debt restructurings. It is intended to assist creditors in determining whether a modification of the terms of a receivable meets the criteria to be considered a troubled debt restructuring, both for purposes of recording an impairment loss and for disclosure of troubled debt restructurings. In evaluating whether a restructuring constitutes a troubled debt restructuring, a creditor must separately conclude that both of the following exist: (a) the restructuring constitutes a concession; and (b) the debtor is experiencing financial difficulties. ASU 2011-02 provides guidance to clarify whether the creditor has granted a concession and whether a debtor is experiencing financial difficulties. The new guidance is effective for interim and annual periods beginning on or after June 15, 2011, and applies retrospectively to restructurings occurring on or after the beginning of the fiscal year of adoption. The adoption of ASU 2011-02 did not have a significant impact on Main Street's financial condition and results of operations.

Inflation

        Inflation has not had a significant effect on our results of operations in any of the reporting periods presented herein. However, our portfolio companies have experienced, and may in the future experience, the impacts of inflation on their operating results, including periodic escalations in their costs for raw materials and required energy consumption.

Off-Balance Sheet Arrangements

        We may be a party to financial instruments with off-balance sheet risk in the normal course of business to meet the financial needs of our portfolio companies. These instruments include commitments to extend credit and involve, to varying degrees, elements of liquidity and credit risk in excess of the amount recognized in the balance sheet. At December 31, 2012, we had a total of $72.4 million in outstanding commitments comprised of (i) seven commitments to fund revolving loans that had not been fully drawn and (ii) five capital commitments that had not been fully called.

Contractual Obligations

        As of December 31, 2012, the future fixed commitments for cash payments in connection with our SBIC debentures for each of the next five years and thereafter are as follows:

        As of December 31, 2012, we had $132.0 million in borrowings outstanding under our Credit Facility. The Credit Facility is scheduled to mature in September 2017. The Credit Facility contains two, one year extension options which could extend the maturity to September 2019.

        Pursuant to the support services agreement with MSCC, the Investment Manager is reimbursed each quarter by MSCC for its cash operating expenses, less fees that the Investment Manager receives from MSC II and third parties, associated with providing investment management and other services to MSCC, certain of its subsidiaries and third parties. For the years ended December 31, 2012 and 2011,

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the expenses reimbursed by MSCC to the Investment Manager and management fees paid by MSC II were $10.7 million and $8.9 million, respectively.

Related Party Transactions

        As discussed further in Note D to the accompanying consolidated financial statements, subsequent to the completion of the Formation Transactions, the Investment Manager is a wholly owned portfolio company of MSCC. At December 31, 2012, the Investment Manager had a receivable of $4.1 million due from MSCC related to operating expenses incurred by the Investment Manager required to support Main Street's business.

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CERTAIN U.S. FEDERAL INCOME TAX CONSEQUENCES

        The following discussion is a summary of certain material U.S. federal income tax consequences relevant to the purchase, ownership and disposition of the Notes, but does not purport to be a complete analysis of all potential tax consequences. The discussion is based upon the Internal Revenue Code of 1986, as amended (the "Code"), the regulations promulgated thereunder by the U.S. Treasury (the "Treasury Regulations"), rulings and pronouncements issued by the Internal Revenue Service (the "IRS"), and judicial decisions, all as of the date hereof and all of which are subject to change at any time. Any such change may be applied retroactively in a manner that could adversely affect a holder of the Notes. We have not sought any ruling from the IRS with respect to the statements made and the conclusions reached in the following discussion, and there can be no assurance that the IRS will agree with such statements and conclusions.

        This discussion does not address all of the U.S. federal income tax consequences that may be relevant to a holder in light of such holder's particular circumstances or to holders subject to special rules, including, without limitation:

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banks, insurance companies and other financial institutions;



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U.S. expatriates and certain former citizens or long-term residents of the United States;



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holders subject to the alternative minimum tax;



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dealers in securities or currencies;



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traders in securities;



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partnerships, S corporations or other pass-through entities;



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U.S. holders (as defined below) whose functional currency is not the U.S. dollar;



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controlled foreign corporations;



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tax-exempt organizations;



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passive foreign investment companies;



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persons holding the Notes as part of a "straddle," "hedge," "conversion transaction" or other risk reduction transaction; and



•

persons deemed to sell the Notes under the constructive sale provisions of the Code.

        In addition, this discussion is limited to persons purchasing the Notes for cash at original issue and at their original "issue price" within the meaning of Section 1273 of the Code (i.e., the first price at which a substantial amount of the Notes are sold to the public for cash). Moreover, the effects of other U.S. federal tax laws (such as estate and gift tax laws) and any applicable state, local or foreign tax laws are not discussed. The discussion deals only with notes held as "capital assets" within the meaning of Section 1221 of the Code.

        If an entity taxable as a partnership holds the Notes, the tax treatment of an owner of the entity generally will depend on the status of the particular owner in question and the activities of the entity. Owners of any such entity should consult their tax advisors as to the specific tax consequences to them of holding the Notes indirectly through ownership of such entity.

        YOU ARE URGED TO CONSULT YOUR TAX ADVISOR WITH RESPECT TO THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO YOUR PARTICULAR SITUATION AS WELL AS ANY TAX CONSEQUENCES OF THE PURCHASE, OWNERSHIP AND DISPOSITION OF THE NOTES ARISING UNDER THE U.S. FEDERAL ESTATE OR GIFT TAX LAWS OR UNDER

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THE LAWS OF ANY STATE, LOCAL, FOREIGN OR OTHER TAXING JURISDICTION OR UNDER ANY APPLICABLE TAX TREATY.

U.S. Holders

        The following is a summary of the material U.S. federal income tax consequences that will apply to you if you are a "U.S. holder" of the Notes. As used herein, "U.S. holder" means a beneficial owner of the Notes who is for U.S. federal income tax purposes:

•

an individual who is a citizen or resident of the United States, including an alien individual who is a lawful permanent resident of the United States or meets the "substantial presence" test under Section 7701(b) of the Code;



•

a corporation or other entity taxable as a corporation created or organized in or under the laws of the United States, any state thereof, or the District of Columbia;



•

an estate, the income of which is subject to U.S. federal income tax regardless of its source; or



•

a trust, if a U.S. court can exercise primary supervision over the administration of the trust and one or more "United States persons" within the meaning of Section 7701(a)(30) of the Code can control all substantial trust decisions, or, if the trust was in existence on August 20, 1996, and it has elected to continue to be treated as a United States person.

Payments of Interest

        Stated interest on the Notes generally will be taxable to a U.S. holder as ordinary income at the time that such interest is received or accrued, in accordance with such U.S. holder's method of tax accounting for U.S. federal income tax purposes.

Sale or Other Taxable Disposition of Notes

        A U.S. holder will recognize gain or loss on the sale, exchange, redemption, retirement or other taxable disposition of a note equal to the difference between the amount realized upon the disposition (less any portion allocable to any accrued and unpaid interest, which will be taxable as interest to the extent not previously included in income) and the U.S. holder's adjusted tax basis in the note. A U.S. holder's adjusted tax basis in a note generally will be equal to the amount that the U.S. holder paid for the note less any principal payments received by the U.S. holder. Any gain or loss will be a capital gain or loss, and will be a long-term capital gain or loss if the U.S. holder has held the note for more than one year at the time of disposition. Otherwise, such gain or loss will be a short-term capital gain or loss. Long-term capital gains recognized by certain non-corporate U.S. holders, including individuals, generally will be subject to a reduced tax rate. The deductibility of capital losses is subject to limitations.

Information Reporting and Backup Withholding

        A U.S. holder may be subject to information reporting and backup withholding when such holder receives interest payments on the Notes held or upon the proceeds received upon the sale or other disposition of such notes (including a redemption or retirement of the Notes). Certain holders generally are not subject to information reporting or backup withholding. A U.S. holder will be subject to backup withholding if such holder is not otherwise exempt and such holder:

•

fails to furnish the holder's taxpayer identification number ("TIN"), which, for an individual, ordinarily is his or her social security number;



•

furnishes an incorrect TIN;

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•

is notified by the IRS that the holder has failed properly to report payments of interest or dividends; or



•

fails to certify, under penalties of perjury, that the holder has furnished a correct TIN and that the IRS has not notified the holder that the holder is subject to backup withholding.

        U.S. holders should consult their tax advisors regarding their qualification for an exemption from backup withholding and the procedures for obtaining such an exemption, if applicable. Backup withholding is not an additional tax, and taxpayers may use amounts withheld as a credit against their U.S. federal income tax liability or may claim a refund if they timely provide certain information to the IRS.

Unearned Income Medicare Contribution

        After December 31, 2012, a tax of 3.8% will be imposed on certain "net investment income" (or "undistributed net investment income", in the case of estates and trusts) received by taxpayers with adjusted gross income above certain threshold amounts. "Net investment income" as defined for United States federal Medicare contribution purposes generally includes interest payments and gain recognized from the sale or other disposition of the Notes. Tax-exempt trusts, which are not subject to income taxes generally, and foreign individuals will not be subject to this tax. U.S. holders should consult their own tax advisors regarding the effect, if any, of this tax on their ownership and disposition of the Notes.

Non-U.S. Holders

        The following is a summary of certain material U.S. federal income tax consequences that will apply to you if you are a "Non-U.S. holder" of the Notes. A "Non-U.S. holder" is a beneficial owner of the Notes who is not a U.S. holder or a partnership for federal income tax purposes. Special rules may apply to Non-U.S. holders that are subject to special treatment under the Code, including controlled foreign corporations, passive foreign investment companies, U.S. expatriates, and foreign persons eligible for benefits under an applicable income tax treaty with the U.S. Such Non-U.S. holders should consult their tax advisors to determine the U.S. federal, state, local and other tax consequences that may be relevant to them including any reporting requirements.

Payments of Interest

        Generally, interest income paid to a Non-U.S. holder that is not effectively connected with the Non-U.S. holder's conduct of a U.S. trade or business is subject to withholding tax at a rate of 30% (or, if applicable, a lower treaty rate). Nevertheless, interest paid on a note to a Non-U.S. holder that is not effectively connected with the Non-U.S. holder's conduct of a U.S. trade or business generally will not be subject to U.S. federal withholding tax provided that:

•

such holder does not directly or indirectly, actually or constructively, own 10% or more of the total combined voting power of all classes of our voting stock;



•

such holder is not a controlled foreign corporation that is related to us through actual or constructive stock ownership and is not a bank that received such note on an extension of credit made pursuant to a loan agreement entered into in the ordinary course of its trade or business; and



•

either (1) the Non-U.S. holder certifies in a statement provided to us or the paying agent, under penalties of perjury, that it is the beneficial owner of the Notes and not a "United States person" within the meaning of the Code and provides its name and address, (2) a securities clearing organization, bank or other financial institution that holds customers' securities in the ordinary course of its trade or business and holds the note on behalf of the Non-U.S. holder

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certifies to us or the paying agent under penalties of perjury that it, or the financial institution between it and the Non-U.S. holder, has received from the Non-U.S. holder a statement, under penalties of perjury, that such holder is the beneficial owner of the Notes and is not a United States person and provides us or the paying agent with a copy of such statement or (3) the Non-U.S. holder holds its note directly through a "qualified intermediary" and certain conditions are satisfied.

        Even if the above conditions are not met, a Non-U.S. holder generally will be entitled to a reduction in or an exemption from withholding tax on interest if the Non-U.S. holder provides us or our paying agent with a properly executed IRS Form W-8BEN claiming an exemption from or reduction of the withholding tax under the benefit of a tax treaty between the United States and the Non-U.S. holder's country of residence or a substantially similar substitute form. A Non-U.S. holder is required to inform the recipient of any change in the information on such statement within 30 days of such change. Special certification rules apply to Non-U.S. holders that are pass-through entities rather than corporations or individuals.

        If interest paid to a Non-U.S. holder is effectively connected with the Non-U.S. holder's conduct of a U.S. trade or business (and, if required by an applicable income tax treaty, the Non-U.S. holder maintains a U.S. permanent establishment to which such interest is attributable), then, the Non-U.S. holder will be exempt from U.S. federal withholding tax, so long as the Non-U.S. holder has provided an IRS Form W-8ECI or substantially similar substitute form stating that the interest that the Non-U.S. holder receives on the Notes is effectively connected with the Non-U.S. holder's conduct of a trade or business in the United States. In such a case, a Non-U.S. holder will be subject to tax on the interest it receives on a net income basis in the same manner as if such Non-U.S. holder were a U.S. holder. In addition, if the Non-U.S. holder is a foreign corporation, such interest may be subject to a branch profits tax at a rate of 30% or lower applicable treaty rate.

Sale or Other Taxable Disposition of Notes

        Any gain realized by a Non-U.S. holder on the sale, exchange, retirement, redemption or other taxable disposition of a note generally will not be subject to U.S. federal income tax unless:

•

the gain is effectively connected with the Non-U.S. holder's conduct of a trade or business in the United States (and, if required by an applicable income tax treaty, the Non-U.S. holder maintains a U.S. permanent establishment to which such gain is attributable); or



•

the Non-U.S. holder is an individual who is present in the United States for 183 days or more in the taxable year of sale, exchange or other disposition, certain conditions are met and the Non-U.S. holder is not eligible for relief under an applicable income tax treaty.

        A Non-U.S. holder described in the first bullet point above will be required to pay U.S. federal income tax on the net gain derived from the sale or other taxable disposition generally in the same manner as if such Non-U.S. holder were a U.S. holder, and if such Non-U.S. holder is a foreign corporation, it may also be required to pay an additional branch profits tax at a 30% rate (or a lower rate if so specified by an applicable income tax treaty). A Non-U.S. holder described in the second bullet point above will be subject to U.S. federal income tax at a rate of 30% (or, if applicable, a lower treaty rate) on the gain derived from the sale or other taxable disposition, which may be offset by certain U.S. source capital losses, even though the Non-U.S. holder is not considered a resident of the United States.

Information Reporting and Backup Withholding

        The amount of interest that we pay to any Non-U.S. holder on the Notes will be reported to the Non-U.S. holder and to the IRS annually on an IRS Form 1042-S, regardless of whether any tax was

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actually withheld. Copies of these information returns may also be made available under the provisions of a specific treaty or agreement to the tax authorities of the country in which the Non-U.S. holder resides. However, a Non-U.S. holder generally will not be subject to backup withholding and certain other information reporting with respect to payments that we make to the Non-U.S. holder, provided that we do not have actual knowledge or reason to know that such holder is a "United States person," within the meaning of the Code, and the holder has given us the statement described above under "Non-U.S. holders—Payments of Interest."

        If a Non-U.S. holder sells or exchanges a note through a United States broker or the United States office of a foreign broker, the proceeds from such sale or exchange will be subject to information reporting and backup withholding unless the holder provides a withholding certificate or other appropriate documentary evidence establishing that such holder is not a U.S. holder to the broker and such broker does not have actual knowledge or reason to know that such holder is a U.S. holder, or the holder is an exempt recipient eligible for an exemption from information reporting and backup withholding. If a Non-U.S. holder sells or exchanges a note through the foreign office of a broker who is a United States person or has certain enumerated connections with the United States, the proceeds from such sale or exchange will be subject to information reporting unless the holder provides to such broker a withholding certificate or other documentary evidence establishing that such holder is not a U.S. holder and such broker does not have actual knowledge or reason to know that such evidence is false, or the holder is an exempt recipient eligible for an exemption from information reporting. In circumstances where information reporting by the foreign office of such a broker is required, backup withholding will be required only if the broker has actual knowledge that the holder is a U.S. holder.

        A Non-U.S. holder generally will be entitled to credit any amounts withheld under the backup withholding rules against the holder's U.S. federal income tax liability or may claim a refund provided that the required information is furnished to the IRS in a timely manner.

Foreign Account Tax Compliance Act

        Legislation enacted in 2010 imposes a United States federal withholding tax of 30% on payments of interest or gross proceeds from the disposition of a debt instrument paid after December 31, 2012 to certain non-U.S. entities, including certain foreign financial institutions and investment funds, unless such non-U.S. entity complies with certain reporting requirements regarding its United States account holders and its United States owners. Pursuant to Treasury Regulations and other Treasury guidance, these rules generally are not effective for payments of interest until January 1, 2014, and, in the case of payments of gross proceeds, until January 1, 2017. In addition, Treasury Regulations state that even after the effective dates the new withholding obligations will not apply to payments on, or with respect to, to obligations that are outstanding on January 1, 2014. Prospective purchasers of the Notes should consult their own tax advisors regarding the new withholding and reporting provisions.

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UNDERWRITING

        Under the terms and subject to the conditions contained in an underwriting agreement dated March     , 2013, the underwriters named below, for whom Keefe, Bruyette & Woods, Inc. is acting as representative, have severally agreed to purchase, and we have agreed to sell to them, the aggregate principal amount of Notes indicated below:

        The underwriting agreement provides that the obligations of the underwriters to pay for and accept delivery of the Notes offered hereby are subject to the approval of certain legal matters by their counsel and to certain other conditions. The underwriters are severally obligated to take and pay for all Notes offered hereby (other than those covered by the underwriters' over-allotment option described below) if any such Notes are taken. We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the Securities Act.

        We expect that delivery of the Notes will be made against payment therefor on or about April     , 2013, which will be the third business day following the date of the pricing of the Notes.

Over-Allotment Option

        We have granted to the underwriters an option, exercisable for 30 days from the date of this prospectus supplement, to purchase up to an additional $                 million aggregate principal amount of the Notes offered hereby at the public offering price set forth on the cover page hereof, less the underwriting discount. The underwriters may exercise this option solely for the purpose of covering over-allotments, if any, made in connection with the offering of the Notes offered hereby. To the extent such option is exercised, each underwriter will become obligated, subject to certain conditions, to purchase approximately the same percentage of such additional Notes as the number set forth next to such underwriter's name in the preceding table bears to the total number of Notes set forth next to the names of all underwriters in the preceding table.

Commissions and Discounts

        An underwriting discount of 3% per Note will be paid by us. This underwriting discount will also apply to any Notes purchased pursuant to the over-allotment option.

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        The following table shows the total underwriting discounts and commissions that we are to pay to the underwriters in connection with this offering. The information assumes either no exercise or full exercise by the underwriters of their over-allotment option.

        The underwriters propose to offer some of the Notes to the public at the public offering price set forth on the cover page of this prospectus supplement and some of the Notes to certain other Financial Industry Regulatory Authority members at the public offering price less a concession not in excess of      % of the aggregate principal amount of the Notes. The underwriters may allow, and the dealers may reallow, a discount not in excess of      % of the aggregate principal amount of the Notes. After the initial offering of the Notes to the public, the public offering price and such concessions may be changed. No such change shall change the amount of proceeds to be received by us as set forth on the cover page of this prospectus supplement.

        The expenses of the offering, not including the underwriting discount, are estimated at $200,000 and are payable by us.

No Sales of Similar Securities

        Subject to certain exceptions, we have agreed not to directly or indirectly, offer, pledge, sell, contract to sell, grant any option for the sale of, or otherwise transfer or dispose of any debt securities issued or guaranteed by the Company that are substantially similar to the Notes or any securities convertible into or exercisable or exchangeable for such debt securities for a period of 30 days after the date of this prospectus supplement without first obtaining the written consent of Keefe, Bruyette & Woods, Inc. This consent may be given at any time without public notice.

Listing

        The Notes are a new issue of securities with no established trading market. We intend to list the Notes on the NYSE. We expect trading of the Notes on the NYSE to begin within 30 days after the original issue date under the trading symbol "MSCA." Currently there is no public market for the Notes.

        We have been advised by the underwriters that they presently intend to make a market in the Notes after completion of the offering as permitted by applicable laws and regulations. The underwriters are not obligated, however, to make a market in the Notes and any such market-making may be discontinued at any time in the sole discretion of the underwriters without any notice. Accordingly, no assurance can be given as to the liquidity of, or development of a public trading market for, the Notes. If an active public trading market for the Notes does not develop, the market price and liquidity of the Notes may be adversely affected.

Price Stabilization and Short Positions

        In connection with the offering, the underwriters may purchase and sell Notes in the open market. These transactions may include over-allotment, covering transactions and stabilizing transactions. Over-allotment involves sales of securities in excess of the aggregate principal amount of securities to be purchased by the underwriters in the offering, which creates a short position for the underwriters. Covering transactions involve purchases of the securities in the open market after the distribution has been completed in order to cover short positions. Stabilizing transactions consist of certain bids or

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purchases of securities made for the purpose of preventing or retarding a decline in the market price of the securities while the offering is in progress.

        The underwriters also may impose a penalty bid. This occurs when a particular underwriter repays to the underwriters a portion of the underwriting discount received by it because the representatives have repurchased Notes sold by or for the account of such underwriter in stabilizing or short covering transactions.

        Any of these activities may cause the price of the Notes to be higher than the price that otherwise would exist in the open market in the absence of such transactions. These transactions may be affected in the over-the-counter market or otherwise and, if commenced, may be discontinued at any time without any notice relating thereto.

Electronic Offer, Sale and Distribution of Notes

        A prospectus in electronic format may be made available on the Internet sites or through other online services maintained by one or more of the underwriters and/or selling group members participating in this offering, or by their affiliates. In those cases, prospective investors may view offering terms online and, depending upon the particular underwriter or selling group member, prospective investors may be allowed to place orders online. The underwriters may agree with us to allocate a limited principal amount of the Notes for sale to online brokerage account holders. Any such allocation for online distributions will be made by the underwriters on the same basis as other allocations.

        Other than the prospectus in electronic format, information contained in any other web site maintained by an underwriter or selling group member is not part of this prospectus supplement or the related registration statement, has not been endorsed by us and should not be relied on by investors in deciding whether to purchase any Notes.

Conflicts of Interest

        Affiliates of Raymond James & Associates, Inc., RBC Capital Markets, LLC and BB&T Capital Markets, underwriters in this offering, act as lenders and/or agents under our Credit Facility. Certain of the net proceeds from the sale of our Notes, not including underwriting compensation, may be paid to such affiliates of Raymond James & Associates, Inc., RBC Capital Markets, LLC and BB&T Capital Markets in connection with the repayment of debt owed under our Credit Facility. As a result, Raymond James & Associates, Inc., RBC Capital Markets, LLC, BB&T Capital Markets and/or their affiliates may receive more than 5% of the net proceeds of this offering, not including underwriting compensation.

        The underwriters and/or their affiliates from time to time provide and may in the future provide investment banking, commercial banking and financial advisory services to us, for which they have received and may receive customary compensation.

        In addition, the underwriters and/or their affiliates may from time to time refer investment banking clients to us as potential portfolio investments. If we invest in those clients, we may utilize net proceeds from this offering to fund such investments, and the referring underwriter or its affiliate may receive placement fees from its client in connection with such financing, which placement fees may be paid out of the amount funded by us.

        The addresses of the underwriters are: Keefe, Bruyette & Woods, Inc., 787 Seventh Avenue, New York, New York 10019; Raymond James & Associates, Inc., 880 Carillon Parkway, St. Petersburg, Florida 33716; RBC Capital Markets, LLC, 3 World Financial Center, 200 Vesey Street, 8th Floor, New York, NY 10281; BB&T Capital Markets, 901 East Byrd Street, Suite 410, Richmond, Virginia 23219; Sanders Morris Harris Inc., 600 Travis, Suite 5800, Houston, Texas 77002; Janney Montgomery Scott LLC, 1717 Arch Street, Philadelphia, Pennsylvania 19103.

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LEGAL MATTERS

        Certain legal matters regarding the Notes offered hereby will be passed upon for us by Sutherland Asbill & Brennan LLP, Washington D.C., and certain legal matters in connection with this offering will be passed upon for the underwriters by Bass, Berry & Sims PLC, Memphis, Tennessee.

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

        The consolidated financial statements, Schedule 12-14 and the schedule of Senior Securities of Main Street Capital Corporation, included in this prospectus supplement and the accompanying prospectus have been so included in reliance upon the reports of Grant Thornton LLP, independent registered public accountants, upon the authority of said firm as experts in giving said reports. Grant Thornton LLP's principal business address is 700 Milan St., Suite 300, Houston, Texas 77002.

AVAILABLE INFORMATION

        We have filed with the SEC a universal shelf registration statement on Form N-2, together with all amendments and related exhibits, under the Securities Act, with respect to our Notes offered by this prospectus supplement. The registration statement contains additional information about us and our Notes being offered by this prospectus supplement.

        We file with or submit to the SEC annual, quarterly and current reports, proxy statements and other information meeting the informational requirements of the Securities Exchange Act of 1934. You may inspect and copy these reports, proxy statements and other information, as well as the registration statement and related exhibits and schedules, at the Public Reference Room of the SEC at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the Public Reference Room by calling the SEC at 1-800-SEC-0330. The SEC maintains an Internet site that contains reports, proxy and information statements and other information filed electronically by us with the SEC, which are available on the SEC's website at www.sec.gov. Copies of these reports, proxy and information statements and other information may be obtained, after paying a duplicating fee, by electronic request at the following e-mail address: publicinfo@sec.gov, or by writing the SEC's Public Reference Section, 100 F Street, N.E., Washington, D.C. 20549.

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AUDITED FINANCIAL STATEMENTS

Index to Financial Statements

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Report of Independent Registered Public Accounting Firm

Board of Directors and Stockholders' of
Main Street Capital Corporation

        We have audited the accompanying consolidated balance sheets of Main Street Capital Corporation (a Maryland corporation) ("the Company"), including the consolidated schedule of investments, as of December 31, 2012 and 2011 and the related consolidated statements of operations, changes in net assets and cash flows for each of three years in the period ended December 31, 2012 and the financial highlights (see Note H) for each of the five years in the period ended December 31, 2012. These financial statements and financial highlights are the responsibility of the Company's management. Our responsibility is to express an opinion on these financial statements and financial highlights based on our audits.

        We conducted our audits in accordance with the standards of the Public Company Accounting Oversight Board (United States). Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements and financial highlights are free of material misstatement. An audit includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements. An audit also includes assessing the accounting principles used and significant estimates made by management, as well as evaluating the overall financial statement presentation. Our procedures included verification by confirmation of securities as of December 31, 2012 and 2011, or by other appropriate auditing procedures where replies were not received. We believe that our audits provide a reasonable basis for our opinion.

        In our opinion, the consolidated financial statements and financial highlights referred to above present fairly, in all material respects, the financial position of Main Street Capital Corporation as of December 31, 2012 and 2011 and the results of their operations and their cash flows for each of the three years in the period ended December 31, 2012, and the financial highlights for each of the five years in the period ended December 31, 2012, in conformity with accounting principles generally accepted in the United States of America.

        We also have audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States), the Company's internal control over financial reporting as of December 31, 2012, based on criteria established in Internal Control—Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission (COSO), and our report dated March 8, 2013 (not separately included herein), expressed an unqualified opinion on the effectiveness of the Company's internal control over financial reporting.

/s/ GRANT THORNTON LLP

Houston, Texas
March 8, 2013

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MAIN STREET CAPITAL CORPORATION

Consolidated Balance Sheets

(in thousands, except shares and per share amounts)

   

The accompanying notes are an integral part of these financial statements

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MAIN STREET CAPITAL CORPORATION

Consolidated Statements of Operations

(in thousands, except per share amounts)

   

The accompanying notes are an integral part of these financial statements

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MAIN STREET CAPITAL CORPORATION

Consolidated Statements of Changes in Net Assets

(in thousands, except shares)

The accompanying notes are an integral part of these financial statements

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MAIN STREET CAPITAL CORPORATION

Consolidated Statements of Cash Flows

(in thousands)

   

The accompanying notes are an integral part of these financial statements

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MAIN STREET CAPITAL CORPORATION

CONSOLIDATED SCHEDULE OF INVESTMENTS

December 31, 2012

(in thousands)