Originally established on 20 June 1935 as Civilian Corps (CCC) Camp SP-9 and "garrisoned" by CCC Company 1921-V and later Company 3357. It was located in the Rock City area of Mount Diablo State Park. Corps members worked under Organized Reserve Corps (now the Army Reserve) officers building structures and improving infrastructure on the park.
In 1943, the camp became
a field training area for the Naval Construction Battalion Replacement
Center at Camp Parks. It was used for mountain warfare training
as well as bridge construction and road building training. The
training area continued to be known as Camp Diablo until it was
closed and returned to the State of California in 1946.