William Cavendish, 5th Duke of Devonshire, KG (December 14, 1748 – July 29, 1811), was the eldest son of the 4th Duke of Devonshire by his wife the heiress Lady Charlotte Boyle, suo jure Baroness Clifford of Lanesborough, who brought in considerable money and estates to the Cavendish family. The 5th Duke is best known for his first wife Georgiana, Duchess of Devonshire.

He was married twice: first, to Lady Georgiana Spencer (1757–1806); second, to Lady Elizabeth Foster, nee Hervey (1759–1824), daughter of the 4th Earl of Bristol, who had been his mistress and his first wife's friend and confidante for more than twenty years.

By his first wife, he had issue one son (the 6th Duke or "The Bachelor Duke", who succeeded him and who died unmarried in 1858), and two daughters: Lady Georgiana Cavendish, later the Countess of Carlisle (wife of the 6th Earl of Carlisle), and Lady Harriet "Hary-O" Cavendish, later The Countess Granville (wife of Lord Granville Leveson-Gower, who was created 1st Earl Granville). Both daughters left descendants. The title of Baron Clifford of Lanesborough has fallen into abeyance between them, who are represented today by the present Earl of Carlisle and the present Earl Granville. However, the dukedom and estates passed eventually to a grandson of a younger brother of the 5th Duke; the 7th Duke had however married a daughter of the 6th Earl of Carlisle, and thus a granddaughter of the 5th Duke and niece of the 6th Duke.

Georgiana Spencer was a socialite who gathered around her a large circle of literary and political friends. She was painted by Thomas Gainsborough and Joshua Reynolds; the Gainsborough painting was disposed of by the 5th Duke and was recovered only much later, after many vicissitudes.

 

By his second wife, Lady Elizabeth Foster, he had no legitimate issue. A son Augustus was given the surname of Clifford and became Sir Augustus Clifford and rose to senior rank in the navy. His descendants eventually died out in the male line in 1895. His daughter by Lady Elizabeth, Caroline, was given a different surname from her brother, St. Jules. Caroline St. Jules married the Hon. George Lamb, a brother of the 2nd Viscount Melbourne (himself married to Lady Caroline Ponsonby, niece of the 1st wife of the 5th Duke). The George Lambs had no issue.

The 5th Duke also had a daughter — Charlotte, given the surname of Williams — by his mistress Charlotte Spencer, the daughter of an indigent clergyman. His first child was born shortly after his marriage to Lady Georgiana Spencer (no relation to his mistress). Charlotte was later married off suitably.

The fifth Duke was closely involved with the nearby spa town of Buxton. He used the profits from his copper mines to transform the town into a replica of Bath, including The Crescent and the Devonshire Royal Hospital.