When war broke out with the United States of America, McKay almost immediately offered his services to the British army. His first major contribution was to make a journey of by canoe from Montreal to the British military post on St. Joseph Island on Lake Huron via York, the provincial capital, in only eight days. The news of the declaration of war he carried, along with orders from Major General Isaac Brock, the Commander in Upper Canada, allowed the British detachment to take the American detachment at Fort Mackinac by surprise. This victory encouraged many Native Americans to side with the British, resulting in further British victories in the west during the following year.