The Mountaineers, an outdoor recreation, education, and conservation organization based in Seattle, Washington, was formed in 1906 to explore the wild areas and mountains surrounding the Puget Sound region. Its first president was Edmond S. Meany, and standing committees were appointed at the club's first meeting in order to take over the work and organization of its two main activities: local and annual outings. The number of standing committees grew with the increasing membership and interests of the club.

Early activities of the Mountaineers consisted of hiking, climbing, skiing, snowshoeing and stage productions by the Mountaineers Players. Today, the club offers even more in the way of activities including bicycling, photography, kayaking, sailing, international trips, and environmental education classes. The Mountaineers work to ensure that wilderness areas are preserved and protected through the actions of their Conservation, Recreation Resources, and Stewardship divisions. Volunteer members track issues, mobilize members, do trail maintenance, work with coalitions, and lobby local and national legislators. The club's mission is to enrich the community by helping people explore, conserve, learn about, and enjoy the lands and waters of the Pacific Northwest.

From the founding of the club until 1980, the highlight of the year was the annual summer outing. These generally were for two or three weeks and involved climbs, both major and minor, as well as hikes. Summer outings were attended by as many as 100 Mountaineers in some years. On most trips, pack trains carried equipment while Mountaineers members traveled on foot; however, a few summer outings were conducted in motor vehicle caravans. Early outings were at various locations throughout the Olympic and Cascade Mountains. As time went on, the trips ranged further afield into the American and Canadian Rockies, Sierra Nevadas, and various national parks of the western United States. The summer outings were discontinued in 1981 when the damage to wilderness areas from large groups of campers was no longer considered sustainable.

In addition to lengthy Summer Outings, The Mountaineers also organized regular trail trips, or day hikes, for members in the Cascade and Olympic Mountains, Puget Sound, and coastal areas.

The Mountaineers Players grew out of "dramatic diversions" performed around the campfire at the club's Rhododendron Preserve on the Kitsap Peninsula. On June 17, 1923 the Mountaineers' first formal production, , took place. Mabel Furry, an active Mountaineer member, persuaded the group to take acting classes with Lois Sandall, who served as the Mountaineers Players director from 1924 through 1936 and again from 1941 through 1950. The Mountaineers built an outdoor theater on a level spot on their property near Chico Creek, Washington. The Forest Theater opened on June 6, 1926 with a production of and has remained one of the oldest active outdoor theaters in the country.

Realizing the critical importance of mountain safety, the club created a climbing course in 1935, led by noted conservationist and outdoorsman Wolf Bauer. Bauer, born in the Bavarian Alps, introduced climbing techniques not generally known in the United States. Mountaineers instructors included Lloyd and Mary Anderson, Jack Hossack, and Philip and Agnes Dickert. The climbing structure Monitor Rock, now called Schurman Rock, was built by the Mountaineers at Seattle's Camp Long in 1939 as a way to help train the public in proper climbing techniques before heading off to the mountains. The club also produced the climbing textbook in 1960, which is still considered the standard textbook of mountaineering. The Seattle Mountain Rescue and Safety Council, later renamed the Mountain Rescue Council, was founded by Mountaineers members Ome Daiber, Otto Trott, and Wolf Bauer. This voluntary group established search and rescue procedures and training classes that became the model for other such programs throughout the country. The group was involved with the production of several instructional films illustrating climbing, search and rescue, and safety techniques, some produced by Bob and Ira Spring, notably , narrated by climber Jim Whittaker.