Fort St. John was originally built at the mouth of Bayou St. John on the shore of Lake Pontchartrain. However, since the fort's construction, the old lakeshore has been filled (late 1920's-early 1930's). The new shoreline is approximately 500 yards further out. As originally built, the fort was approximately 50 feet across and held five gun emplacements.

Bayou St. John was at the height of its importance as a transportation artery in 1808 when the present fort was built. Fort St. John and the bayou shared this importance until the third decade of the nineteenth century. By that time the Spanish were gone from the Florida Parishes and the bayou's importance as a transportation route had declined. In 1823 the fort was decommissioned and sold to resort developers in the area. They renamed the area "Spanish Fort"

It was included in the National Register of Historic Places in 1983

Source: Louisiana Register of Historic Places