 In 1793, he was commissioned an ensign in the 66th Regiment of Foot, and served at Gibraltar, in Flanders, in the East Indies in 1798, and under Baird during the invasion of Egypt.In 1806, Duff commanded the centre column in the attack on Buenos Aires.He was appointed colonel of the 92nd Regiment of Foot in 1823, transferring to 37th Regiment of Foot in 1831.