Barré joined the 32nd Regiment of Foot as an ensign in 1746. The regiment was based in Flanders during the War of the Austrian Succession, and Barré gained his initial army experience prior to the end of the conflict in 1748. He continued to serve, and was promoted to lieutenant in 1755, and captain in 1756. During the French and Indian War, he served under his patron General James Wolfe on the Rochefort expedition of 1757, when he first met Lord Shelburne, and afterwards in Canada where he was appointed adjutant-general, fighting at both Louisbourg (1758) and Quebec (1759). In 1759, he was promoted to major, but the rank applied only during his service in America. In the Quebec expedition, in which Wolfe was killed, Barré was severely wounded by a bullet in the cheek and lost the use of his right eye. He was among the group gathered around the dying Wolfe, which was immortalized in Benjamin West's celebrated picture.