William Henry Cavendish Cavendish-Bentinck, 3rd Duke of Portland, KG, PC, FRS (14 April 1738 – 30 October 1809) was a British Whig and then a Tory politician during the late Georgian era. He served as chancellor of the University of Oxford (1792–1809) and as Prime Minister of Great Britain (1783) and then of the United Kingdom (1807–1809). The gap of 26 years between his two terms as prime minister is the longest of any British prime minister. He was also the fourth great-grandfather of King Charles III through his great-granddaughter Cecilia Bowes-Lyon, Countess of Strathmore and Kinghorne.

Portland was known before 1762 by the courtesy title Marquess of Titchfield. He held a title for every degree of British nobility: duke, marquess, earl (Earl of Portland), viscount (Viscount Woodstock), and baron (Baron Cirencester). He was the leader of the Portland Whigs faction, which broke with the Whig leadership of Charles James Fox and joined with William Pitt the Younger in the wake of the French Revolution.

William Henry, Lord Titchfield, was born on 14 April 1738 at Bulstrode Park in Buckinghamshire.^[1] He was the eldest son of William Bentinck, 2nd Duke of Portland and "the richest woman in great Britain" Lady Margaret Cavendish-Harley and inherited many lands from his mother and his maternal grandmother,^[2][3][4] who was the daughter of John Holles, 1st Duke of Newcastle.^[5] He was educated at Westminster School and Christ Church, Oxford, where he graduated MA in 1757.^[6][5]

In December 1757, the nineteen-year-old Lord Titchfield was sent to study under Lord Stormont for a year in Warsaw, he was accompanied by Stormont's secretary Benjamin Langlois. Stormont was to superintend all expenditures on his equipage, while Benjamin Langlois was to hire local masters and direct the studies of teenage Titchfield. The books he directed to read were ancient history, modern history, general law.^[7]

In 1759, he travelled with Benjamin Langlois through Germany to Italy, spent a year in Turin, and went on to Florence. When Stormont was appointed ambassador to Vienna in 1763, Langlois went with him as Secretary of the embassy.^[8]

On 8 November 1766, Portland married Lady Dorothy Cavendish, a daughter of William Cavendish, 4th Duke of Devonshire and Lady Charlotte Boyle. They were parents of six children:

• William Bentinck, 4th Duke of Portland (24 June 1768 – 27 March 1854).
• Lord William Henry Cavendish-Bentinck (14 September 1774 – 17 June 1839).
• Lady Charlotte Cavendish-Bentinck (2 October 1775 – 28 July 1862).^[9] Married Charles Greville, and they had three sons: Charles Cavendish Fulke Greville, Algernon Greville, and Henry William Greville (1801–1872), and a daughter, Harriet (1803–1870) m. Francis Egerton, 1st Earl of Ellesmere.^[10]
• Lady Mary Cavendish-Bentinck (13 March 1779 – 6 November 1843).^[11]
• Lord Charles Bentinck (20 May 1780 – 28 April 1826).^[12] Ancestor of the 6th and 7th dukes of Portland and Queen Mother Elizabeth.
• Lord Frederick Cavendish-Bentinck (2 November 1781 – 11 February 1828) married Lady Mary Lowther (died 1863), daughter of William Lowther, 1st Earl of Lonsdale, 16 September 1820; had issue: George Cavendish-Bentinck, ancestor of the 8th and 9th dukes of Portland.
• A stillborn baby, birthed at Burlington House on 20 October 1786.^[13]

Portland was elected to sit in the Parliament of Great Britain for Weobley in 1761 before he entered the House of Lords after he succeeded his father as Duke of Portland the next year. He was associated with the aristocratic Whig Party of Lord Rockingham and served as Lord Chamberlain of the Household in Rockingham's first government (1765–1766).^[5]

He died on 30 October 1809 at Burlington House, Piccadilly, after an operation for the stone, and was buried at St Marylebone Parish Church, London.^[19]

He had lived expensively: with an income of £17,000 a year (worth £577,000 in 2005),^[20] he had debts at his death computed at £52,000 (£1.76 million in 2005),^[20] which were paid off by his succeeding son by selling off some property, including Bulstrode Park.^[21]

Along with Sir Robert Peel, Lord Aberdeen, Benjamin Disraeli, Marquess of Salisbury, Sir Henry Campbell-Bannerman, Bonar Law and Neville Chamberlain, he was the first of eight British prime ministers to die while his direct successor was in office.

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The Portland Vase of Roman glass was given its name because it was owned by Portland at his family residence at Bulstrode Park.

Portland Parish, in Jamaica, was named after him. The Titchfield School, founded in 1786, is in the parish and is also named in his honour. The school's crest is derived from his personal crest.

Two major streets in Marylebone are named after him: Portland Place and Great Portland Street. Both were built on land that he once owned.

North Bentinck Arm and South Bentinck Arm were named for the Bentinck family by George Vancouver in 1793, along with other names on the British Columbia Coast, such as Portland Canal and Portland Channel.

Portland Bay in Victoria, Australia was named in 1800 by the British navigator James Grant. The city of Portland is located on the bay.

The department of Manuscripts and Special Collections, The University of Nottingham holds a number of papers relating to him. His personal and political papers (Pw F) are part of the Portland (Welbeck) Collection, and the Portland (London) Collection (Pl) contains his correspondence and official papers, especially in series Pl C.

The Portland Estate Papers held at Nottinghamshire Archives also contain items relating to the 3rd Duke's properties.

The Portland Collection^[22] of fine and decorative art includes pieces owned and commissioned by him, including paintings by George Stubbs.