Located on the west bank of the Penobscot River in Prospect, Maine, in an area known as the Penobscot Narrows, Fort Knox is one of the best preserved fortifications on the New England seacoast. The Fort has many architectural features present only to itself, as well as a rich history behind its cannon batteries.

Maine was repeatedly involved in northeast border disputes with British Canada, and the area between Castine and the rich lumber city of Bangor was invaded and occupied by the British during the American Revolution and the War of 1812. Despite the Webster-Ashburton Treaty of 1842, Fort Knox was established in 1844 to protect the Penobscot River valley against a possible future British naval incursion.

The Fort was designed by Chief Engineer Joseph Totten with a number of other engineers serving as superintendents of construction from 1844 - 1869, among them Isaac Ingalls Stevens and Thomas L. Casey. The Fort was named for Major General Henry Knox, America's first Secretary of War, who was born in Boston but retired to Thomaston, Maine in 1796. The Fort garrisoned its first troops from 1863 to 1866. These troops were mostly volunteers undergoing training before being sent to their active posts and included members of the celebrated 20th Maine. Troops were also briefly stationed at the Fort during the Spanish American war in 1898, but never saw military action.