Form 8-K

8-K — WHITE MOUNTAINS INSURANCE GROUP LTD

Accession: 0001628280-26-022818

Filed: 2026-04-01

Period: 2026-04-01

CIK: 0000776867

SIC: 6331 (FIRE, MARINE & CASUALTY INSURANCE)

Item: Other Events

Item: Financial Statements and Exhibits

Documents

8-K — wtm-20260401.htm (Primary)

EX-99.1 (wtmex991-basesixpressrelea.htm)

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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

April 1, 2026

Date of Report (Date of earliest event reported)

WHITE MOUNTAINS INSURANCE GROUP, LTD.

(Exact name of registrant as specified in its charter)

Bermuda

(State or other jurisdiction of

incorporation or organization)

1-8993

(Commission file number)

94-2708455

(I.R.S. Employer Identification No.)

23 South Main Street, Suite 3B, Hanover, New Hampshire 03755

(Address of principal executive offices)

(603) 640-2200

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Title of each class Trading Symbol(s) Name of each exchange on which registered

Common Shares, par value $1.00 per share WTM New York Stock Exchange

WTM.BH Bermuda Stock Exchange

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933(§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter). Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. o

ITEM 8.01 Other Events.

White Mountains Insurance Group, Ltd. (NYSE: WTM) (“White Mountains”) announced that White Mountains Partners LLC (“WTM Partners”) has completed the acquisition of a majority interest in BaseSix Systems LLC, a low voltage electrical systems integrator. White Mountains equity investment was approximately $97 million.

A copy of the press release announcing the acquisition is attached hereto as Exhibit 99.1.

ITEM 9.01. FINANCIAL STATEMENTS AND EXHIBITS.

(d) Exhibits

99.1 Press Release, dated April 1, 2026

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

WHITE MOUNTAINS INSURANCE GROUP, LTD.

April 1, 2026 By:

/s/ MICHAELA J. HILDRETH

Michaela J. Hildreth

Managing Director and

Chief Accounting Officer

EX-99.1

Filename: wtmex991-basesixpressrelea.htm · Sequence: 2

Document

Exhibit 99.1

PRESS RELEASE

WHITE MOUNTAINS PARTNERS ANNOUNCES ACQUISITION

OF BASESIX

NEW YORK, New York, April 1, 2026 – White Mountains Partners (“WMP”), a White Mountains operating company, announced today that it has acquired a majority interest in BaseSix Systems LLC (“Basesix” or the “Company”), a provider of building systems integration and aftermarket service. Founded in 2018 and headquartered in Marietta, Georgia, Basesix provides the design, installation, retrofit, maintenance, and repair of mission critical, low voltage building systems for commercial and institutional customers across the fire & life safety, network & wireless, security & access control, and audio-visual disciplines.

John Daly, White Mountains Partners’ CEO and Managing Partner, said, “Basesix was founded with a vision to build a best-in-class multidisciplinary systems integration platform. Under the leadership of Co-Founders, Robb Borden (CEO) and Chris Atwell (Executive Vice President), the Company’s senior management team, and Basesix’s divisional and functional Champions, the Company has achieved significant organic growth and earned an exceptional reputation for on-time, on-budget, high quality work for the most complex integration requirements. We look forward to collaborating with Basesix to help enable the Company’s next chapter of growth.”

Robb Borden, Co-Founder and CEO, commented, "I would like to express my sincere gratitude to all Basesix employees. Your unwavering dedication and hard work have been the driving force behind building the Company into what it is today. We have now reached an inflection point in Basesix’s evolution where the next logical step is to join forces with a strong financial partner to pursue the significant growth opportunities ahead, including opening new offices and strategic acquisitions. We sought a capital partner who shared our core principles, unwavering culture, and strategic vision. We are excited to partner with WMP to build upon our proven model of success.”

Morgan, Lewis & Bockius LLP acted as legal counsel to WMP. Deloitte Corporate Finance LLC acted as financial advisor and Miller & Martin PLLC acted as legal counsel to Basesix.

ABOUT BASESIX

Basesix simplifies building systems by transforming complex technologies into user-friendly integrated solutions. With a broad brush, Basesix makes buildings, campuses, and environments safer, smarter, and simpler by combining our people and their talents with products designed for a purpose. Our customers supply the need or desire, and we take it from there. Additional information is available on Basesix’s website located at www.basesix.com.

ABOUT WHITE MOUNTAINS PARTNERS

White Mountains Partners is a wholly-owned business unit of White Mountains Insurance Group, Ltd. (NYSE: WTM) and provides first institutional capital to family, founder, and entrepreneur-owned businesses in the essential services, light industrial and specialty consumer sectors. Additional information is available on White Mountains Partners’ website located at www.wtmpartners.com.

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