The group was known initially as The Mountaineers, Auxiliary to the Mazamas, because many of the organizers were associated with the Mazamas mountaineering club of Oregon. The organization adopted a constitution in 1907, and that November the name was reduced to just "The Mountaineers." There were 150 charter members, including University of Washington geologist (and husband of future Seattle mayor Bertha Landes) Henry Landes, historian Edmond S. Meany (1862-1935), and the photographer Asahel Curtis (1874-1941). Climbers such as Jim Whittaker (b. 1929), the first American to reach the summit of Mount Everest, got their training at Seattle's Mountaineers Club.
In 1909, the Everett Branch was formed, followed in 1912 by the Tacoma Branch. Then branches were formed in Olympia, Bellingham, and Wenatchee. Most recently, two new branches have been established on the Kitsap Peninsula and in the Snoqualmie foothills. In 2000, the central membership was officially declared the Seattle Branch. As of that year, the club had 15,000 members and was the third largest such organization in the United States. The administration headquarters for the entire club resides in Seattle.
The non-profit club sponsors activities such as hiking, backpacking, climbing, skiing and snowboarding, whitewater and sea kayaking, alpine scrambling, sailing, nature study, biking, snowshoeing, and foreign travel.
The club has a book publishing division, and also participates in lobbying for conservation efforts.