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. ==Prime Minister: 1756–1757==