Mordaunt was the second son of John Mordaunt, Viscount Mordaunt and Frances Powlett and educated at Westminster School. He joined the Army as a cornet in the Royal Horse Guards from 1726 to 1736. In 1745, during the Jacobite Rebellion, he rejoined the Army to serve as the lieutenant colonel of the Duke of Kingston's Regiment of Light Horse, which he commanded at the Battle of Culloden.