Exhibit 99.1 CONTACT: Rob Seelig (603) 640-2212 WHITE MOUNTAINS REPORTS SECOND QUARTER RESULTS HAMILTON, Bermuda (August 7, 2025) - White Mountains Insurance Group, Ltd. (NYSE: WTM) reported book value per share of $1,804 as of June 30, 2025, an increase of 3% for both the second quarter of 2025 and the first six months of 2025, including dividends. Manning Rountree, CEO, commented, "BVPS was up 3% in the quarter. We had sound results at our operating companies and good investment returns. Ark produced an 8 ...