Supplemental Disclosures to the Proxy Statement

The following disclosure supplements and restates the fifth paragraph on page42 of the Proxy Statement/Prospectus in the section captioned “The Merger—Background of the Merger”:

Upon reaching out to Party 3 subsequent to the May1, 2017 board meeting, representatives of RBCCM were informed that Party 3 was no longer in a position to pursue an acquisition. On May9, 2017 and May17, 2017, Bank Mutual executed confidentiality agreements with Party 2 and with Associated, respectively; the confidentiality agreements included standstill provisions, including restrictions on asking to be released from the standstill, binding on Associated and Party 2.

The following disclosure supplements and restates the fourth paragraph beginning on page43 of the Proxy Statement/Prospectus in the section captioned “The Merger—Background of the Merger”:

On June27, 2017, Bank Mutual’s chairman and management team met telephonically with representatives of each of Quarles& Brady and RBCCM to review the indications of interest. At the conclusion of the meeting, Bank Mutual’s chairman and management team directed representatives of RBCCM to reach out to Associated and Party 2 and ask them to provide a revised offer, which should reflect their best and final offer, by June30, 2017. As part of that request, Party 2 was asked to clarify the potential board role, and Associated was asked if it would consider such a role for a Bank Mutual director. Following this telephonic meeting and throughout the period that Bank Mutual was engaging in discussions with Associated and the other potential acquirers generally, Bank Mutual’s chairman regularly kept directors apprised of developments.

The following disclosure supplements and restates last paragraph beginning on page43 and continuing onto page44 of the Proxy Statement/Prospectus in the section captioned “The Merger—Background of the Merger”:

On July5, 2017, the Bank Mutual board met to discuss the two indications of interest. At that meeting, representatives of RBCCM provided financial analyses of each of the offers, and a representative of Quarles& Brady provided an extended review of the directors’ fiduciary and related duties in considering the proposals as well as legal aspects of the offers, the comments made by Associated and Party 2 on the draft merger agreement and matters described in the section entitled “The Merger—Bank Mutual’s Reasons for the Merger; Recommendation of the Bank Mutual Board of Directors.” After careful and extended deliberation, including questions for representatives of each of RBCCM and Quarles& Brady, and including an extended executive session without executive officers or Mr.Crowley Jr. present, the Bank Mutual board unanimously elected to proceed with a potential acquisition transaction, and to move forward toward the negotiation and signing of a definitive merger agreement with Associated. Associated and Party 2 were then informed of the board’s decision.

The following disclosure supplements and restates the second-to-last paragraph on page44 of the Proxy Statement/Prospectus in the section captioned “The Merger—Background of the Merger”:

During the period between July5, 2017 and July20, 2017, Bank Mutual’s chairman regularly kept directors apprised of developments, including by circulating drafts of the various transaction documents and other materials prepared by the management of Bank Mutual and its legal and financial advisors relating to the proposed transaction. Also, after material business terms were settled, Associated reached understandings with certain individuals as to their roles post-merger; see the section entitled “Interests of Bank Mutual’s Directors and Executive Officers in the Merger.”

The following disclosure supplements and restates the second full paragraph beginning on page50 of the Proxy Statement/Prospectus and continuing onto page51 in the section captioned “The Merger—Opinion of Bank Mutual’s Financial Advisor—Bank Mutual Financial Analysis—Public Company Analysis”:

In this analysis, RBCCM compared, among other things, (i)multiples of implied price per share of common equity to (a)TBV, (b)NTM EPS and (c)estimated 2018 EPS and (ii)CDP. The list of selected companies, related multiples and percentages and resultant high, median and low multiples and percentages for such selected companies and for Bank Mutual are as follows:

Selected Companies

(1)NM refers to Not Meaningful. Ratios of estimated Price/NTM EPS were greater than 30.0x and, as a result, were considered not meaningful and were not included in the calculations of high, median and low.

(2)NA refers to Not Available. Figures for estimated NTM EPS and estimated 2018 EPS were not available and, as a result, no Price/NTM EPS or Price/2018 EPS ratios were included in the calculations of high, median and low.

The following disclosure supplements and restates the last paragraph beginning on page51 of the Proxy Statement/Prospectus and continuing onto page52 in the section captioned “The Merger—Opinion of Bank Mutual’s Financial Advisor—Bank Mutual Financial Analysis—Selected Transaction Analysis”:

In this analysis, RBCCM compared, among other things, (i)multiples of implied price per share of common equity to (a)TBV and (b)NTM EPS and (ii)CDP. The list of selected transactions, related multiples and percentages and resultant high, median and low multiples and percentages for such selected transactions and for Bank Mutual are as follows:

(1)NA refers to Not Available. Figures for estimated NTM EPS were not available and, as a result, no Price/NTM EPS or Price/2018 EPS ratios were included in the calculations of high, median and low.

(2)NM refers to Not Meaningful. Ratios of estimated Price/NTM EPS were greater than 30.0x and, as a result, were considered not meaningful and were not included in the calculations of high, median and low.

The following disclosure supplements and restates the last paragraph beginning on page52 of the Proxy Statement/Prospectus and continuing onto page53 in the section captioned “The Merger—Opinion of Bank Mutual’s Financial Advisor—Bank Mutual Financial Analysis—Dividend Discount Analysis”:

RBCCM performed the Bank Mutual DDA using discount rates ranging from 10.0% to 13.0%, based on an estimated cost of equity for Bank Mutual using its professional judgment in applying the capital asset pricing model (“CAPM”), inclusive of an equity size premium, and a terminal value at the end of the forecast period, using terminal multiples ranging from 14.0x to 18.0x estimated 2022 EPS. The terminal multiples for the Bank Mutual DDA were selected based on a review of the historical EPS multiples for the selected companies referred to above. The Bank Mutual DDA indicated the following implied per share reference ranges as compared to implied per share merger consideration:

The following disclosure supplements and restates the last paragraph on page53 of the Proxy Statement/Prospectus in the section captioned “The Merger—Opinion of Bank Mutual’s Financial Advisor—Associated Financial Analysis—Public Company Analysis”:

In this analysis, RBCCM compared, among other things, multiples of implied price per share of common equity to (i)TBV, (ii)NTM EPS and (iii)estimated 2018 EPS. The list of selected companies, related multiples and the resultant high, median and low multiples for such selected companies and for Associated are as follows:

Selected Companies

The following disclosure supplements and restates the third paragraph on page54 of the Proxy Statement/Prospectus in the section captioned “The Merger—Opinion of Bank Mutual’s Financial Advisor—Associated Financial Analysis—Dividend Discount Analysis”:

RBCCM performed the Associated DDA using discount rates ranging from 10.0% to 13.0% based on an estimated cost of equity for Associated using its professional judgment in applying CAPM, inclusive of equity size premium, and a terminal value at the end of the forecast period, using terminal multiples ranging from 14.0x to 18.0x estimated 2022 EPS. The terminal multiples were selected based on a review of the historical EPS multiples for the selected public companies referred to above. The Associated DDA indicated the following implied per share value references range, as compared to the July19, 2017 closing price of Associated common stock:

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Forward Looking Statements

This communication contains “forward-looking statements” within the meaning of the federal securities laws, including Section27A of the Securities Act of 1933, as amended, and Section21E of the Securities Exchange Act of 1934, as amended. These forward-looking statements may include: management plans relating to the proposed transaction; the expected timing of the completion of the proposed transaction; the ability to complete the proposed transaction; the ability to obtain the required regulatory, shareholder or other approvals; any statements of the plans and objectives of management for future operations, products or services, including the execution of integration plans relating to the proposed transaction; any statements of expectation or belief; projections related to certain financial metrics; and any statements of assumptions underlying any of the foregoing. Forward-looking statements are typically identified by words such as “believe,” “expect,” “anticipate,” “intend,” “seek,” “plan,” “will,” “would,” “target,” “outlook,” “estimate,” “forecast,” “project” and other similar words and expressions or negatives of these words. Forward-looking statements are subject to numerous assumptions, risks and uncertainties, which change over time and are beyond our control. Forward-looking statements speak only as of the date they are made. Neither Associated nor Bank Mutual assumes any duty and does not undertake to update any forward-looking statements. Because forward-looking statements are by their nature, to different degrees, uncertain and subject to assumptions, actual results or future events could differ, possibly materially, from those that Associated or Bank Mutual anticipated in its forward-looking statements, and future results could differ materially from historical performance. Factors that could cause or contribute to such differences include, but are not limited to, those included under Item 1A “Risk Factors” in Associated’s Annual Report on Form10-K, those included under Item 1A “Risk Factors” in Bank Mutual’s Annual Report on Form10-K, those disclosed in Associated’s and Bank Mutual’s respective other periodic reports filed with the SEC, as well as the possibility that expected benefits of the proposed transaction may not materialize in the time frame expected or at all, or may be more costly to achieve; the proposed transaction may not be timely completed, if at all; that prior to the

completion of the proposed transaction or thereafter, Associated’s and Bank Mutual’s respective businesses may not perform as expected due to transaction-related uncertainty or other factors; that the parties are unable to successfully implement integration strategies related to the proposed transaction; that required regulatory, shareholder or other approvals are not obtained or other customary closing conditions are not satisfied in a timely manner or at all; reputational risks and the reaction of the companies’ shareholders, customers, employees or other constituents to the proposed transaction; and diversion of management time on merger-related matters. These risks, as well as other risks associated with the proposed transaction, are more fully discussed in the proxy statement/prospectus included in the registration statement on FormS-4 filed with the SEC in connection with the proposed transaction. While the list of factors presented here is, and the list of factors presented in the registration statement on FormS-4 is, considered representative, no such lists should be considered to be a complete statement of all potential risks and uncertainties. Unlisted factors may present significant additional obstacles to the realization of forward-looking statements. For any forward-looking statements made in this communication or in any documents, Associated and Bank Mutual claim the protection of the safe harbor for forward-looking statements contained in the Private Securities Litigation Reform Act of 1995. Associated and Bank Mutual annualized, pro forma, projected and estimated numbers are used for illustrative purposes only, are not forecasts and may not reflect actual results.

Important Additional Information and Where to Find It

In connection with the proposed merger, Associated filed with the SEC a Registration Statement on FormS-4 that includes a Proxy Statement of Bank Mutual and a Prospectus of Associated, as well as other relevant documents concerning the proposed transaction. The Registration Statement has been declared effective and the Proxy Statement/Prospectus has been mailed to shareholders of Bank Mutual. This communication does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval. SHAREHOLDERS OF BANK MUTUAL CORPORATION ARE URGED TO READ THE REGISTRATION STATEMENT AND THE PROXY STATEMENT/PROSPECTUS REGARDING THE MERGER AND ANY OTHER RELEVANT DOCUMENTS FILED WITH THE SEC, AS WELL AS ANY AMENDMENTS OR SUPPLEMENTS TO THOSE DOCUMENTS, BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION.

A free copy of the Proxy Statement/Prospectus, as well as other filings containing information about Associated and Bank Mutual, may be obtained at the SEC’s Internet site (http://www.sec.gov). You will also be able to obtain these documents, free of charge, from Associated Banc-Corp at www.associatedbank.com under the heading “About” and then under the heading “Investor Relations” and then under “SEC Filings” or from Bank Mutual Corporation at www.bankmutual.com/bank-mutual-corporation/ under the heading “Financial& SEC Reports.” Copies of the Proxy Statement/Prospectus can also be obtained, free of charge, by directing a request to Associated Banc-Corp, 433 Main Street, Green Bay, Wisconsin 54301, Attention: Investor Relations, Telephone: (920) 491-7059 or to Bank Mutual Corporation, 4949 West Brown Deer Road, Milwaukee, Wisconsin 53223, Attention: Michael W. Dosland, Telephone: (414) 354-1500.

Participants in the Solicitation

Associated, Bank Mutual, and certain of their respective directors, executive officers and employees may be deemed to be participants in the solicitation of proxies in respect of the proposed transaction. Information regarding Associated’s directors and executive officers is available in its definitive proxy statement for its most recent annual meeting, which was filed with the SEC on March14, 2017, and certain of its Current Reports on Form8-K.

Information regarding Bank Mutual’s directors and executive officers is available in its definitive proxy statement for its most recent annual meeting, which was filed with SEC on March8, 2017, and certain of its Current Reports on Form8-K. Other information regarding the participants in the proxy solicitation and a description of their direct and indirect interests, by security holdings or otherwise, is contained in the proxy statement/prospectus relating to the merger of Associated and Bank Mutual and other relevant materials filed with the SEC. Free copies of this document may be obtained as described in the preceding paragraph.

About BANK MUTUAL CORPORATION (NASDAQ:BKMU)

Bank Mutual Corporation is a savings and loan holding company. The Company owns Bank Mutual (the Bank), a federally-chartered savings bank. The Company operates through community banking segment. The Bank is engaged in the business of community banking, which includes attracting deposits from and making loans to the general public and private businesses, as well as governmental and non-profit entities. In addition to deposits, the Bank obtains funds through borrowings from the Federal Home Loan Bank (FHLB) of Chicago. These funding sources are used to originate loans, including commercial and industrial loans, multi-family residential loans, non-residential commercial real estate loans, one- to four-family loans, home equity loans and other consumer loans. The Bank also purchases and/or participates in loans from third-party financial institutions and is a seller of residential loans in the secondary market. It also invests in mortgage-related and other investment securities.