The British under General Jeffery Amherst captured the fort in the 1759 Battle of Ticonderoga. In this confrontation 11,000 British troops, using emplaced artillery, drove off the token garrison of 400 Frenchmen. The French, in withdrawing, used explosives to destroy what they could of the fort and spiked or dumped cannons that they did not take with them. Although the British worked in 1759 and 1760 to repair and improve the fort, it was not part of any further significant action in the war. After the war, the British garrisoned the fort with a small number of troops and allowed it to fall into disrepair

On May 10, 1775, less than one month after the Revolutionary War was ignited with the battles of Lexington and Concord, the British garrison of 48 soldiers was surprised by a small force of Green Mountain Boys, along with militia volunteers from Massachusetts and Connecticut, led by Ethan Allen and Benedict Arnold. With the capture of the fort, the Patriot forces obtained a large supply of cannons and other armaments, much of which Henry Knox transported to Boston during the winter of 1775–1776. Ticonderoga's cannons were instrumental in ending the siege of Boston when they were used to fortify Dorchester Heights.

General Burgoyne led 7,800 British and Hessian forces south from Quebec in June 1777. After occupying nearby Fort Crown Point without opposition on June 30, he prepared to besiege Ticonderoga. Burgoyne realized the tactical advantage of the high ground, and had his troops haul cannons to the top of Mount Defiance. Faced with bombardment from the heights (although no shots had yet been fired), General St. Clair ordered Ticonderoga abandoned on July 5, 1777. Burgoyne's troops moved in the next day, with advance guards pursuing the retreating Patriot Americans.

The fort’s earlier French history is central to understanding the later American seizure. Built as Fort Carillon near the southern end of Lake Champlain, it guarded a water and portage corridor between the Hudson valley and Canada. In 1758 French defenders repelled a much larger British assault nearby, but they abandoned the fort the following year as British artillery approached. The later British name, Ticonderoga, became familiar during the Revolution.

The fort’s significance varied with control of the route. The captured guns carried to Boston helped the American army pressure the British to evacuate the city in 1776. When Burgoyne advanced from Canada in 1777, artillery placed on Mount Defiance rendered the American position untenable and forced another withdrawal. The present visitor site combines archaeological remains, restored fortification, collections and interpretive events; it should not be mistaken for an entirely untouched eighteenth-century garrison.