British soldier, an outstanding staff officer who was the friend and trusted colleague of John Churchill, 1st Duke of Marlborough. (Marlborough, John Churchill, 1st Duke of, Marquess Of Blandford, Earl Of Marlborough, Baron Churchill Of Sandridge, Lord Churchill Of Eyemouth, Reichsfürst)

The son of a Dublin barrister, Cadogan began his military career in 1690. In 1702 he was made quartermaster general to Marlborough, who had just been appointed commander of the allied armies arrayed against the French (War of the Spanish Succession, 1701–14). Cadogan served beside Marlborough in the victories at Blenheim, in Bavaria (1704), and at Ramillies, in present-day Belgium (1706), and he led a cavalry regiment in numerous engagements with the French. In 1709 he was promoted to lieutenant general.

When Marlborough was dismissed by Queen Anne in 1711, Cadogan retired from the army. He became involved in intrigues to secure—by force if necessary—the succession for the house of Hanover, and after the Hanoverian monarch George I came to power in 1714, Cadogan again received military promotions. He crushed a rebellion by Jacobites (supporters of Stuart claims to the throne) in 1716 and two years later was granted an earldom. Nevertheless, by allying with George's Hanoverian advisers, Cadogan left himself open to attacks by the opposition; hence, by 1720 he had lost most of his political influence.