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The end of the Revolutionary War brought significant change, with Florida being ceded back to Spain after the 1783 Treaty of Paris. Consequently, the fort’s name was reverted back to Castillo de San Marcos. The early 19th century saw changes in governance as the United States acquired Florida through the Adams-Onis Treaty in 1821, solidifying control by renaming the fort Fort Marion.
The fort continued to play its role through various conflicts, including the Civil War, where it served as a supply depot and held prisoners. Ultimately, it transitioned to a national monument in 1924 and became a part of the National Park Service in 1933.