Founded April 11, 1934, the Washington County Historical Society is a private, non-profit educational institution with about 700 members. There are two membership meetings held annually during the late summer and late winter. The society operates two museum sites, each with two structures. The society's research library has been growing since that first meeting in 1934. It is housed in the reconstructed carriage house behind the Warden's House Museum. The WCHS endowment fund was established in 1984 to provide a source of funding for everyday administrative and operating expenses.
In the early years of the society, the membership meetings were held at the Stillwater Public Library. The first society president was Edward E. Bloomquist, who was also the county school superintendent. The first donation to the society was the book History of Washington County and the St. Croix Valley by Warner and Foote.
In 1941, the Society purchased the Warden's House Museum in Stillwater from the state. It is one of the state's oldest buildings as well as its second oldest continuously operating house museum. The Society leased the Hay Lake School for a museum in 1974 and purchased it from the Forest Lake school district in 1978. The school's neighbor, the Johannes Erickson Log House, was donated to the Society in the early 1980s, moved to the site and restored. All three buildings are on the National Register of Historic Places. The historic family-owned Boutwell Cemetery west of Stillwater was purchased in 1978.
In 1996 the Society reconstructed the Carriage House that originally stood behind the Warden's House. In 2001 climate control was added and the textile collection and archives were moved into the new facility. In March 2002, the Society opened a new museum space in the Boutwells Landing development in Oak Park Heights.