Stuart was appointed captain in the 56th Regiment of Foot on 1 November 1755. He first saw active service at the siege of Louisbourg (present-day Nova Scotia) under Lord Amherst in 1758. On 9 May of the same year he was promoted to the rank of major, and in 1761 was present with Colonel Morgan's regiment at the reduction of Belle Île. During the course of the expedition he acted as quartermaster-general, and in consequence obtained the rank of lieutenant-colonel.