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.

[3] Horace Walpole described him as "a favourite by descent of the Old Whigs"[4] and as "errant [a] bigot to the Pelham faction as ever Jacques Clément was to the Jesuits".

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's administration secured increased money for the war, troops were sent to America and a Militia Act was passed.

[11] 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.

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.

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".

The wedding was held at Carlton House, the then residence of the Dowager Lady Burlington, situated between St James' Park and Pall Mall, by special licence on the 28 March, 1748.

[18] Paul Langford said Devonshire was "eminently sensible and highly respected", and that his death "left a marked gap in the ranks" of the opposition and "effectively destroyed a generation in the 'Old Whig' leadership".

He was endowed with the qualities—devotion to friends and duty, patriotism, and unswerving integrity—which made him the ideal sounding board and factotum among the prominent politicians of his day.

Unlike Pitt or Fox he lacked a brilliant mind, and his diary provides evidence of devotion to king, country, and duty rather than quickness of intellect.