The Carnegie Library building was originally built to be a free library in 1904/05. It was the first library in San Luis Obispo County. Funds for the construction of the Carnegie Library building were provided by a grant of $10,000 from steel tycoon Andrew Carnegie, who funded the establishment of some 3,000 libraries worldwide. The Museum building was designed by W.H. Weeks, an architect from Watsonville, California, and was built by the construction firm of Stephens and Maino of San Luis Obispo. The structure’s sandstone was brought from Los Berros, near Arroyo Grande. The granite in the building was quarried from nearby Bishop’s Peak and brought into the city on a spur of the narrow gauge Pacific Coast Railway. The Museum is the only building in San Luis Obispo County built with both of these local stones. The building served as the public library for the City of San Luis Obispo until 1955, when a new library was built at the corner of Palm and Morro Streets.
When a search for a permanent home for the Museum was undertaken in 1953, the newly formed San Luis Obispo County Historical Society received assistance from the San Luis Obispo City Council and the County Board of Supervisors. An agreement was struck regarding the old library building, and in 1956, the San Luis Obispo County Historical Museum was opened to the public.
Members of the Society had long dreamed of creating a county historical museum where local artifacts could be properly displayed and archived. This dream was realized on October 4, 1955 when the City of San Luis Obispo granted a renewable 50 year lease for the historic Carnegie Library in downtown San Luis Obispo to the Society for it to be used as a museum. The Society has spent the next fifty years building the largest and finest archive of historical artifacts of any museum located in San Luis Obispo County.
Thanks to annual financial assistance from the County of San Luis Obispo, the Museum has been in continuous operation since 1906. The building and the museum were closed temporarily during a seismic retrofit and general upgrades in July 1999. Maino Construction firm performed the retrofit work. The work was done by Tom Maino, grandson of the original Maino Construction firm owner who built the building. The Maino family also donated the money for the sign in front of the building today. The museum was reopened in May, 2001 with new rotating exhibits and a new public research room.