Stewart, born in Scotland, was a career British Army officer. Initially commissioned into the 37th Regiment of Foot, he was promoted to lieutenant colonel of the 3rd Regimen of Foot in 1775. Stewart arrived in Charleston, South Carolina with the regiment in June of 1781. He took command of the British forces in Charleston after the commander, Lord Rawdon, fell ill and returned to Britain. Because of this, Stewart was in command at the Battle of Eutaw Springs. After the battle, he returned to Charleston with his forces for the remainder of the war. Following the American Revolution, Stewart continued his successful military career, rising to major general in 1790 and serving in Parliament until his death in 1794.