Union forces occupied the hill during the Civil War. In 1861, General Benjamin Butler and the 6th Massachusetts Regiment, along with members of Cook's Light Artillery, occupied the hill. The army constructed fortifications, in part as a defensive response to the presence of numerous Confederate sympathizers in Baltimore. The site became known as Fort Federal Hill during this time.
Sold to the City of Baltimore in 1880, Federal Hill became a public park. A monument to Colonel George Armistead, in charge of Fort McHenry during the Battle of Baltimore, was raised on the hill in 1882.