He joined the Army, serving during the War of the Spanish Succession where he saw action at the Battles of Oudenarde and Malplaquet. His career benefited from his brother's close connection to the Army's Captain General the Duke of Marlborough. He rose, by 1715, to the rank of Lieutenant-Colonel in the Coldstream Guards. He was promoted Brigadier-General in 1735, Major-General in 1739, Lieutenant-General in 1745 and full General in 1761. Atterbury describes Cadogan as "a bold, bad, boistrous, blustering, bloody, booby.".