In 1961, a group of eleven local community members gathered together to discuss the creation of an organization that would be charged with the preservation of the history of the Kittitas Valley. The community members were descendants of early pioneers of Kittitas County.

On September 20, 1961, the Kittitas County Historical Society was officially formed with the primary goal of establishing a space in which to display a small collection of historic items. They were soon able to set up this display in a single room at the Morris Sorenson Memorial County Hospital, and hosted their first open house and tours on May 6, 1962.

In 1974, the Historical Society was informed that the space in the Morris Sorenson County Hospital would no longer be available. The Historical Society began to negotiate a deal to purchase the Cadwell Building. Since the Historical Society did not have the funds at first to purchase nor relocate to the Cadwell Building, Kittitas County Commissioners and the Historical Society worked out an agreement for the new museum to house the then-recently donated Rollinger Brothers’ Rock and Mineral Collection in exchange for a rental amount to pay towards the mortgage. The Historical Society was able to quickly pay off the mortgage and maintained the collection until 2012 when the Kittitas County Commissioners approved the transfer of ownership of the Rollinger Brothers Rock and Mineral Collection to the Kittitas County Historical Museum.

In the 1980s, the Historical Society sought to increase its funding source, and ultimately size, by purchasing the buildings that neighbored the Cadwell Building. In 1981, the Historical Society purchased the McCullough Addition, an extension of the west side of the original 1889 building. The museum then underwent a major renovation in 1998 that would accommodate the rising number of visitors, researchers, and volunteers through the addition of display, storage, and working spaces.

In 2008, the museum underwent its most recent renovation. The main entrance was moved from its previous location to its current location to provide better access for all visitors to the museum. The front desk was also relocated.

Today, the Kittitas County Historical Museum offers displays including an Antique Doll Collection, Native American Bag and Basket Collection, Antique Automobile display, items from early Kittitas County Businesses, Military History items, and much more!