It was also never attacked during the American Revolution, though it was used from 1777 to 1783 as a POW camp. Up to a thousand British and German soldiers captured after the Battles of Saratoga and Yorktown in 1777 and 1781 respectively were quartered at Fort Frederick. The fort was auctioned off in 1791 and abandoned. Then in December 1861 the 1st Maryland Infantry, Potomac Horse Brigade garrisoned the fort and used it as a gun emplacement to protect the C&O Canal and B&O Railroad. On December 25, 1861, a Confederate raiding party attacked the 1st Maryland's H Company at the fort. Fort Frederick was then abandoned in February 1861, the military usefulness of it at an end, though it was occupied by a 12th Illinois Cavalry picket in October of that year.
In 1922, the property was acquired by Maryland as Maryland's first state park. The fort had deteriorated, but discovery of the original plans allowed a complete reconstruction. The Civilian Conservation Corps rebuilt it during the 1930s. Two of the three barracks buildings have been restored. Fort Frederick, intended as a shelter, has been used as a patrol base, prison camp, and gun platform, and now serves as a tourist destination, and is used for the occasional historical reenactment.