The nucleus of Fort Mason was a private property owned by John C. Frémont, the world-famous explorer of the US west, who also spearheaded the conquest of California from Mexico, and ran as the first presidential nominee of the extant Republican Party in 1856.
Appointed a Major General in the Union army at the start of the Civil War, Frémont's repeated serious conflicts with President Lincoln led him to resign by late 1862.