He joined the British Army in April 1756, serving as an ensign in the 28th Regiment of Foot. He saw service in the French and Indian War under the command of Jeffery Amherst at the 1758 siege of Louisbourg, and in 1759 under the command of James Wolfe at the Siege of Quebec. He was appointed brigade-major in July 1760 and served as a staff officer in the campaign against Montreal under James Murray. On 16 September 1762, St. Leger was promoted to major in the 95th Regiment of Foot.