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Liam P. Caffrey Chief Executive Officer April 1, 2026 Dear Fellow Shareholder: We are pleased to invite you to the Annual General Meeting (“AGM”) of Members of White Mountains Insurance Group, Ltd., to be held on Thursday, May 21, 2026 beginning at 8:00 a.m. Atlantic Time. This meeting will take place at Rosewood Bermuda Hotel, 60 Tucker’s Point Drive, Hamilton Parish, Bermuda. We will confine this year’s AGM to a shareholder vote on proposals set forth in the accompanying Proxy Statement and on such other matters properly brought before the meeting. At the AGM, you will be asked to consider and vote on the following proposals as further described in the Proxy Statement: 1. Election of four directors to Class II with terms ending in 2029. 2. Approval of the advisory resolution on executive compensation. 3. Approval of the appointment of PricewaterhouseCoopers LLP as the Company’s Independent Registered Public Accounting Firm for 2026. Your vote is important. You can ensure that your common shares are properly represented at the meeting by completing, signing, dating, and promptly returning your proxy card in the enclosed envelope, or following the instructions for telephone and internet voting. Members who hold their shares in a brokerage account, an employee benefit plan, or through a nominee will need to follow the instructions on their proxy card, which may include options to vote their shares by telephone or over the internet. White Mountains will hold its annual Investor Information Meeting on Friday, June 5, 2026 at 10:00 a.m. Eastern Time at the Mandarin Oriental Hotel at 80 Columbus Circle, New York, NY 10023. The meeting will take place in the Mandarin Ballroom, 36 th Floor. Members are welcome to participate either in person or via webcast. The Company will issue a press release with meeting details and webcast instructions in advance of the meeting. This information will also be posted to the Company’s website at www.whitemountains.com. On behalf of all of us here at White Mountains, thank you for being an owner of the Company. Respectfully submitted, Liam P. Caffrey