Fort Johnson is a state-owned historic site of military and political significance located on the northeast point of James Island in Charleston County, South Carolina. Fort Johnson was strategically important during the colonial era, as it was located on the banks of the Ashley River. The fort was named after Sir Nathaniel Johnson, who served as the Goveā¦Fort Johnson is a state-owned historic site of military and political significance located on the northeast point of James Island in Charleston County, South Carolina. Fort Johnson was strategically important during the colonial era, as it was located on the banks of the Ashley River. The fort was named after Sir Nathaniel Johnson, who served as the Governor of Carolina from 1703 to 1709. It was the site of the first raising of the South Carolina state flag in 1775. The magazine was built in 1765 and is a brick structure that measures 27 feet long and 20 feet wide.