Marin County Fire Battalion Chief Mike Giannini described the damage as minimum to moderate and said it involved office furniture and other office-type materials.
The fire was reported around 4:30 a.m. Tuesday.
There were no injuries.
Fire officials believe an electric motor in an exhaust fan in the ceiling was to blame.
Skywalker Ranch sits on about 6,000 acres and serves as Lucas's filmmaking outpost.