William Cadogan Cadogan, 1st Earl (1675-1726), British soldier, was the son of Henry Cadogan, a Dublin barrister, and grandson of Major William Cadogan (1601-1661), governor of Trim. The family has been credited with a descent from Cadwgan, the old Welsh prince. Cadogan began his military career as a cornet of horse under William III. at the Boyne, and, with the regiment now known as the 5th (Royal Irish) Lancers, made the campaigns in the Low Countries. In the course of these years he attracted the notice of Marlborough. In 1701 Cadogan was employed by him as a staff officer in the complicated task of concentrating the grand army formed by contingents from multitudinous states, and Marlborough soon made the young officer his confidential staff officer and right-hand man. His services in the campaign of 1701 were rewarded with the colonelcy of the famous "Cadogan's Horse" (now the 5th Dragoon Guards). As quartermaster-general, it fell to his lot to organize the celebrated march of the allies to the Danube, which, as well as the return march with its heavy convoys, he managed with consummate skill. At the Schellenberg he was wounded and his horse shot under him, and at Blenheim he acted as Marlborough's chief of staff.

Soon afterwards he was promoted brigadier-general, and in 1705 he led "Cadogan's Horse" at the forcing of the Brabant lines between Wange and Elissem, capturing four standards. He was present at Ramillies, and immediately afterwards was sent to take Antwerp, which he did without difficulty. Becoming major-general in 1706, he continued to perform the numerous duties of chief staff officer, quartermaster-general and colonel of cavalry, besides which he was throughout constantly employed in delicate diplomatic missions. In the course of the campaign of 1707, when leading a foraging expedition, he fell into the hands of the enemy but was soon exchanged. In 1708 he commanded the advanced guard of the army in the operations which culminated in the victory of Oudenarde, and in the same year he was with Webb at the action of Wynendael. On the 1st of January 1709 he was made lieutenant-general. At the siege of Menin in this year occurred an incident which well illustrates his qualifications as a staff officer and diplomatist. Marlborough, riding with his staff close to the French, suddenly dropped his glove and told Cadogan to pick it up.

This seemingly insolent command was carried out at once, and when Marlborough on the return to camp explained that he wished a battery to be erected on the spot, Cadogan informed him that he had already given orders to that effect. He was present at Malplaquet, and after the battle was sent off to form the siege of Mons, at which he was dangerously wounded. At the end of the year he received the appointment of lieutenant of the Tower, but he continued with the army in Flanders to the end of the war. His loyalty to the fallen Marlborough cost him, in 1712, his rank, positions and emoluments under the crown. George I. on his accession, however, reinstated Cadogan, and, amongst other appointments, made him lieutenant of the ordnance. In 1715, as British plenipotentiary, he signed the third Barrier Treaty between Great Britain, Holland and the emperor. His last campaign was the Jacobite insurrection of 1715-1716. At first as Argyle's subordinate (see Coxe, Memoirs of Marlborough, cap. cxiv.), and later as commander-in-chief, General Cadogan by his firm, energetic and skilful handling of his task restored quiet and order in Scotland. Up to the death of Marlborough he was continually employed in diplomatic posts of special trust, and in 1718 he was made Earl Cadogan, Viscount Caversham and Baron Cadogan of Oakley. In 1722 he succeeded his old chief as head of the army and master-general of the ordnance, becoming at the same time colonel of the 1st or Grenadier Guards. He sat in five successive parliaments as member for Woodstock. He died at Kensington in 1726, leaving two daughters, one of whom married the second duke of Richmond and the other the second son of William earl of Portland.

Readers of Esmond will have formed a very unfavourable estimate of Cadogan, and it should be remembered that Thackeray's hero was the friend and supporter of the opposition and General Webb. As a soldier, Cadogan was one of the best staff officers in the annals of the British army, and in command of detachments, and also as a commander-in-chief, he showed himself to be an able, careful and withal dashing leader.

He was succeeded, by special remainder, in the barony by his brother, General Charles Cadogan (1691-1776), who married the daughter of Sir Hans Sloane, thus beginning the association of the family with Chelsea, and died in 1776, being succeeded in turn by his son Charles Sloane (1728-1807), who in the year 1800 was created Viscount Chelsea and Earl Cadogan. His descendant George Henry, 5th Earl Cadogan (b. 1840), was lord privy seal from 1886 to 1892, and lord-lieutenant of Ireland from 1895 to 1902.