Cavendish, William, fourth Duke of Devonshire 1720-1764, first lord of the treasury, and prime minister from November 1756 to May 1757, at the beginning of the seven years' war, eldest son of William Cavendish, third duke of Devonshire, K.G., and lord-lieutenant of Ireland from 1737 to 1744, was born in 1720. He was elected to the House of Commons as M.P. for Derbyshire in 1741, directly he came of age, and was re-elected in 1747, and on 20 March 1748 married Charlotte, baroness Clifford of Londesborough in her own right, only daughter and heiress of Richard Boyle, earl of Burlington and Cork, who brought him Lismore Castle and large estates in Ireland. This marriage greatly increased his political importance, and on 13 June 1751 the Marquis of Hartington, as he was then styled, was summoned to the House of Lords in his father's barony as Lord Cavendish of Hardwicke, and in the following month he was made master of the horse and sworn of the privy council. In March 1754 the Marquis of Hartington was made lord-treasurer of Ireland, and on 27 March constituted lord-lieutenant and general-governor of that island, and on 5 Dec. 1755 he succeeded his father as fourth duke of Devonshire. In Ireland he displayed no very great political ability, but succeeded very happily in pleasing all parties and making himself extremely popular. In 1756 the seven years' war broke out, and all England demanded that Mr. Pitt should be placed at the head of affairs; he absolutely declined to serve under the Duke of Newcastle, who had been prime minister ever since the death of his brother, Henry Pelham, in 1754, and the influence of the great whig families was strong enough to prevent the king from at once making Pitt prime minister. In this dilemma Devonshire was summoned from Ireland, and asked to become prime minister, with Pitt as secretary of state to manage the war. He was eminently a fit man for the post; his rank as a born leader of the whigs, his experience in the House of Commons, and his popularity in Ireland all recommended him, and he was sworn in as first lord of the treasury on 16 Nov. 1756. He was not, however, a success in his new capacity; his leader of the House of Commons, Sir Thomas Robinson, only excited the risibility of Pitt, and Pitt himself soon recognised the necessity of making up his differences with the Duke of Newcastle. In May 1757, therefore, Devonshire, who had been made lord-lieutenant of Derbyshire on 15 Dec. 1756, and a K.G. on 13 Nov. 1756, resigned to the Duke of Newcastle, and was appointed lord-chamberlain of the household, a post which he held until 1762. His health was rapidly declining, and he died at Spa on 3 Oct. 1764, at the age of forty-four.
Collins's Peerage, and the histories of England during the eighteenth century.
Contributor: H. M. S. [Henry Morse Stephens]