He was ordered by Gen. Guy Carleton to delay the American advance on Montreal, and Quebec. He was in command of 662 men of the 26th Regiment, engineers, Canadians, and Indians. They held Fort St. Jean, at Saint-Jean-sur-Richelieu, Quebec, in 1775 when it came under siege by American General Richard Montgomery, and resisted multiple attacks by troops under command of Montgomery and Philip Schuyler. His troops held the fort in hopes of being rescued by reinforcements for two months before surrendering on the third of November 1775.
Preston is mentioned in a Fort Saint-Jean plaque erected in 1926 by Historic Sites and Monuments Board of Canada at the Royal Military College Saint-Jean. "Constructed in 1743 by M. de Léry under orders from Governor la Galissonnière. This post was for all the military expeditions towards Lake Champlain. In August 31, 1760, Commandant de Roquemaure had it blown up in accordance with orders from the Governor de Vaudreuil in order to prevent its falling into the hands of the English. Rebuilt by Governor Carleton, in 1773. During the same year, under the command of Major Charles Preston of the 26th Regiment, it withstood a 45 day siege by the American troops commanded by General Montgomery."