William Cavendish, 4th Duke of Devonshire (8 May 1720 – 2 October 1764), styled Lord Cavendish before 1729, and Marquess of Hartington between 1729 and 1755, was a British Whig statesman and nobleman who was briefly nominal Prime Minister of Great Britain. He was the first son of William Cavendish, 3rd Duke of Devonshire and his wife, Catherine (née Hoskins). He is also a great-great-great-great-great-grandfather of King Charles III through the king's maternal great-grandmother, Cecilia Bowes-Lyon, Countess of Strathmore and Kinghorne.
In October 1762, George III suspected that Devonshire and Thomas Pelham-Holles, 1st Duke of Newcastle, were plotting against him. He had mistaken a visit by Devonshire as a tender of resignation, but George refused to see him in person. Four days later, George personally struck out Devonshire's name from the list of Privy Councillors.
Cavendish's health declined during the 1760s. He visited the town of Spa in the Austrian Netherlands for treatment in its mineral baths, but he died there in October 1764. Dying at the age of 44 years and 147 days, Devonshire remains the shortest-lived British prime minister. Devonshire was buried at Derby Cathedral.
The eldest of four sons of William Cavendish, 3rd Duke of Devonshire, and Catherine (née Hoskins), he was baptised on 1 June 1720 at St Martin's-in-the-Fields in London. He was possibly educated privately at home before going on a grand tour in France and Italy, accompanied by his tutor, in 1739–40.
He was elected MP for Derbyshire in 1741 and 1747. Devonshire was a supporter of Sir Robert Walpole and, after Walpole's fall from power, of the Pelhams. Henry Pelham wrote to Devonshire's father that he was "our mainstay among the young ones, of themselves liable to wander".
Horace Walpole described him as "a favourite by descent of the Old Whigs" and as "errant [a] bigot to the Pelham faction as ever Jacques Clément was to the Jesuits".
He had been offered the post of governor to the Prince of Wales but he declined.
Pelham appointed him Master of the Horse, a post he held until 1755. To take it he left the House of Commons for the Lords by writ of acceleration as Baron Cavendish and joined the Privy Council.
In 1753, as Marquess of Hartington, he acted as intermediary in arranging a Treasury pension for the journalist James Ralph, editor of the opposition weekly The Protester; after an introduction by David Garrick he brokered terms under which Ralph withdrew from political writing.
Devonshire supported the Duke of Newcastle after Henry Pelham's death in 1754 and was Lord Lieutenant of Ireland from 2 April 1755 until 3 January 1757 in Newcastle's administration. In April 1755 he was one of the Lords Justices of the realm upon the King's absence in Hanover.
Devonshire succeeded his father as Duke of Devonshire in December 1755 after his death.
The Seven Years' War was going badly for Britain under the leadership of the Duke of Newcastle and when he resigned in October 1756, George II eventually asked Devonshire to form an administration. Devonshire accepted on the condition that his tenure would last only until the end of the parliamentary session. Devonshire believed his duty to the King required an administration capable of prosecuting the war successfully.
Devonshire was given the Garter and appointed First Lord of the Treasury (most historians consider him prime minister during this service) in November 1756, and he served as First Lord until May 1757 in an administration effectively run by William Pitt. Devonshire's administration secured increased money for the war, troops were sent to America and a Militia Act was passed.
The administration was eventually brought down for a variety of reasons including the opposition of George II and the alleged mishandling of the trial and execution of Admiral John Byng. It was replaced by the Pitt–Newcastle ministry headed by the Duke of Newcastle and including Pitt, Henry Fox and the Duke of Bedford. This government steered Britain through most of the Seven Years' War, leading the country to ultimate victory.
Devonshire was Lord Chamberlain in Newcastle's government (with a seat in the inner Cabinet) and his relations with him were close. George II died in October 1760 and was succeeded by his grandson George III, who was suspicious of Devonshire and Newcastle. When Newcastle resigned in May 1762 Devonshire said that he would rarely attend Lord Bute's councils.
When, in October, George III requested that he attend a Cabinet meeting on peace terms, Devonshire declined, claiming he had inadequate knowledge of the subject.
On 28 October, travelling from Kew to London, the King overtook Devonshire and Newcastle's coach in the belief that the two dukes were plotting and that Devonshire was coming to tender his resignation. He was in fact passing through London to Chatsworth House and had come to give his leave to the King. When Devonshire arrived, George III refused to see him, as he later wrote: "I ordered the page to tell him I would not see him, on which he bid him ask me with whom he should leave his wand...I said he would receive my orders...On the Duke of Devonshire's going away he said to the page, God bless you, it will be very long before you see me here again At a meeting of the Privy Council four days later the King personally struck out Devonshire's name from the list of Privy Councillors. In the opinion of one of his biographers, John Brooke, "Few things in King George III's long life show him in so poor a light".
Devonshire resigned his Lord Lieutenancy of Derbyshire in solidarity with Newcastle and Rockingham when they were dismissed from their Lord Lieutenancies.