The present Fort Ontario is located on the site occupied by previous versions of older forts. The British first constructed Fort of the Six Nations in 1755, an eight-pointed wooden stockade for 300 men. The French destroyed this fort in 1756, as well as destroying Fort Oswego (built 1727) on the opposite bank of the Oswego River, and Fort George (built 1755) on high ground behind Fort Oswego. The British rebuilt Fort Ontario in 1759 out of earth and timber, with casemates, barracks, a dry moat, and several outer redoubts, garrisoned by 500 men. Fort Oswego was not rebuilt. The British abandoned the area in 1777 after several Patriot victories in the state, and Patriot forces then destroyed the empty fort. The British returned yet again in 1782 to rebuild the fort, and held it until 1796, turning it over to the United States in accordance with the Jay Treaty of 1794. The Americans did little with the fort, and it was destroyed by the British in 1814.
The fort was not rebuilt again until 1839. Designed for 120 men, it included a Powder Magazine, Enlisted Men's Barracks, Officers' Quarters #1 and #2, and a Storehouse. The East and West Guardhouses were built during the American Civil War, along with several improvements to the outer wall and outer earthworks. The fort was abandoned as an artillery post in 1901.
Fort Ontario was regarrisoned as an infantry training center in 1903, and many new brick and wood-frame structures were built outside of the old fort. The moat and outer earthworks were removed to allow further construction. Fort Ontario became Army General Hospital #5 from 1917 to 1921. The old fort itself was left abandoned until 1928, when the Enlisted Men's Barracks was converted to the Officers' Club, and other buildings were used as apartments for junior officers. The post was used as an Emergency Refugee Center between 1944 and 1946 for European Holocaust victims, the only such center in the United States. Fort Ontario was finally abandoned as a military post in 1946, and developed as a state park in 1949. The majority of the new construction has since been removed. The fort has been restored to the 1868 period, with exhibits and living history demonstrations.
(excerpts from the Fort Ontario SHS brochure)