He became Commanding Officer of the 21st Regiment of Foot in 1721 and inherited his father's title the following year. He became a Scottish representative peer in 1723. In 1732 he took over command of the 25th Regiment of Foot. He fought at the Battle of Dettingen in 1743 and, having transferred to the Horse Grenadier Guards, at the Battle of Rocoux in 1744. In 1751 he transferred to the military staff in Ireland and in 1758 he became Commander-in-Chief, Ireland. His home, Leslie House in the village of Leslie, was seriously damaged by fire in 1763. He died at his home in 1767.