Cavendish, William (1720–64), marquess of Hartington, 4th duke of Devonshire , lord lieutenant of Ireland, was baptised in London 1 June 1720. He was the eldest son of William Cavendish, 3rd duke, and Catherine, daughter and heir of John Hoskins of Oxted, Surrey. As marquess of Hartington (the courtesy title he inherited on his father becoming 3rd duke of Devonshire in 1729), he served as MP for Derbyshire (1741–51). He married (March 1748) Lady Charlotte Elizabeth Boyle (d. December 1754), only surviving child and heiress of Richard Boyle, 3rd earl of Burlington, thus securing Lismore Castle, Co. Waterford, and other large estates in Ireland, and increasing the political importance of the Devonshire family. Cavendish was raised to the British house of lords (13 June 1751) as Lord Cavendish of Hartington and appointed master of the horse, a notable honour. He was sworn a privy councillor in July. He became 4th duke of Devonshire on the death of his father in December 1755.
In April 1755 he was appointed lord lieutenant and general governor of Ireland and sworn in on 5 May. He was also made governor of Co. Cork. The decision of the prime minister, the duke of Newcastle, to appoint Hartington was appropriate for a number of reasons. Not only was he politically close to Newcastle, he had extensive land holdings in Ireland and his father had been lord lieutenant (1737–44). He was related or connected to the leading political families in Ireland. Henry Boyle (qv), speaker of the house of commons, was his recently deceased wife's uncle and the wives of both John Ponsonby (qv) and his brother William (qv) were his sisters. The Ponsonbys’ political opponent, the earl of Kildare (qv) (later duke of Leinster), regarded him benignly because he was a political ally at Westminster of Henry Fox, Kildare's brother-in-law.
With his chief secretary, Henry Seymour Conway (qv), Hartington was expected to deal with the aftermath of the money bill dispute of 1753, which his predecessor, the duke of Dorset (qv), had been unable to resolve. This dispute had seen Speaker Boyle and his followers side with the opposition to defeat a money bill intended to apply a surplus in the revenue to reduction of the national debt. Working through his chief secretary, whose diplomatic skills were of considerable importance in finding a resolution to the 1753 impasse, he secured an agreement between the factions on the terms of the address to the king at the opening of the parliamentary session in October; this agreement was followed early in 1756 by Henry Boyle's resignation as speaker to become earl of Shannon, with Boyle's great rival John Ponsonby taking the speakership in his place. The viceroy's decision not to appoint the primate George Stone (qv) as a lord justice, when the time came for him to return to England, was intended to conciliate both Boyle and Kildare, and it was indeed the latter who was sworn in as a lord justice, along with Brabazon Ponsonby (qv), 1st earl of Bessborough (d. 1758), and the 1st Viscount Jocelyn (qv), on 11 May 1756 when Devonshire returned to England.
The outbreak of the Seven Years War necessitated his return to England (May 1756). He became first lord of the treasury (November), but faced great difficulties and struggled in the role, resigning in May 1757. He then served as lord chamberlain of the household (1757–62), but six months after Newcastle's resignation he was dismissed from office, with George III personally striking his name from the list of privy councillors. He was made FRS (1761) and FSA (1762). With his health failing, Devonshire retired from politics. He died 3 October 1764 at Spa, and was buried in his family's crypt at All Saints', Derby. Devonshire was succeeded by the eldest son of his three sons, William, as 5th duke. His daughter Dorothy married the 3rd duke of Portland (qv).