During the American Revolutionary War, a detachment from the 3rd New York Regiment destroyed the fort in July, 1778, after the British abandoned it.

After a period of disuse, new construction was undertaken in part because of tensions with Great Britain as well as to check smuggling activities between Canada and the United States.

[5] In 1940, the fort was refurbished as an induction center for new conscripts with 60 new buildings for 3,000 men, but it was used instead as a base for several National Guard anti-aircraft units.

As America entered World War II, Fort Ontario was repurposed again as a training center for African American military police.

[1] In 2010, Fort Ontario was one of the state-funded historical sites named by Governor David Paterson as being potentially unfunded because of the budget crisis faced by New York State.

The present fortifications are built on a pentagonal plan with five bastions, similar to the layout of Fort Jackson, Louisiana.

The stone-faced scarp walls of the bastions were also pierced with loopholes for rifle fire, which are of varying design, reflecting the preferences of the different engineers overseeing the construction.

Originally, a ravelin protected the side of the fort facing Lake Ontario and mounted further heavy guns.