Exhibit 32.1

PRINCIPAL EXECUTIVE OFFICER

CERTIFICATION PURSUANT TO

18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the quarterly report on Form 10-Q of White Mountains Insurance Group, Ltd. (the “Company”), for the period ending June 30, 2018 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, G. Manning Rountree, Chief Executive Officer of the Company, certify, pursuant to 18 U.S.C. §1350, as adopted pursuant to §906 of the Sarbanes-Oxley Act of 2002, that to my knowledge:

• The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and,

• The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company as of and for the periods presented in the Report.

/s/ G. Manning Rountree

Chief Executive Officer

(Principal Executive Officer)

August 7, 2018

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