Built in 1865, the Schoolhouse was the first publicly owned school in San Diego County. The building was a one-room, wood-frame, shingle-roofed structure with a ten foot high ceiling. A pot-bellied iron stove heated the room, and a water bucket and dipper provided the only indoor plumbing. All eight grades were taught in the single room.

In 1873, the school was moved to Taylor and Whitman Streets, and a two-story school was erected there. It was later razed; the original school was returned and reconstructed on this site. The Schoolhouse was operated by the San Diego Historical Days Association as the "Mason Street School" until 2013 when it was given to Old Town San Diego State Historic Park.