William Henry Cavendish Bentinck (1738-1809), was a notable statesman. He was MP for Weobley in 1761-62, when he succeeded his father as the 3rd Duke of Portland, and entered the House of Lords as a Whig. From 1764 he took up the family sinecure as a trustee of the British Museum; in 1765-66 he served as Lord Chamberlain; in 1766 he married Lady Dorothy Cavendish, daughter of the 4th Duke of Devonshire, and became a Fellow of the Royal Society; in 1775 he became a Fellow of the Society of Antiquaries; in 1782 he served as Lord Lieutenant of Ireland; in 1783 he served briefly as Prime Minister; in 1794 he was made a Knight of the Garter, and began his service as Home Secretary, which lasted until 1801; and in 1807-9 he served again as Prime Minister, resigning only shortly before his death.

Being active in political life the 3rd Duke of Portland mainly resided in London, either at his wife's family townhouse, Devonshire House, or that of his own family which was located at Whitehall. This plot had been leased from the crown by the Portland family since 1696, as their apartments within the palace had been severely damaged by the fire in 1691. Repairs and alterations were made to the house in 1754-54 by Stiff Leadbetter (d1766) for the 2nd Duke of Portland, but this work must have been inadequate as the 3rd Duke commissioned Adam to make designs for a grand palace on a site on New Cavendish Street, facing Mansfield Street. This was presumably intended to emphasis his political position.

Mansfield Street had been laid out by the Adam brothers in c1770, and it was following this that Adam made designs to replace Portland's meagre house at Whitehall with the new palace on New Cavendish Street. Nothing of Adam's designs for Portland House was executed, most likely as the Duke could not afford the construction works. His fortune was severely limited by his mother's portion, and his electoral campaigns were costly. Indeed, on his death the 3rd Duke was £500,000 in debt. The plot on New Cavendish Street was abandoned by the end of 1776 when a pair of terraced houses were built there to designs by John Johnson (1732-1814) for William Udney and Sir Charles Bampfylde. Portland even surrendered the lease of his plot at Whitehall to the Crown in 1805 and the house was demolished.

Literature:
A.T. Bolton, The architecture of Robert and James Adam, 1922, Volume II, p. 94, Index pp. 42, 84; Survey of London, Volume XIII, 1930, pp. 185-88; B. Weinreb, and C. Hibbert, The London encyclopaedia, 1983, p. 633; J. Ingamells, A dictionary of British and Irish travellers in Italy 1701-1800, 1997, p. 944; D. King, The complete works of Robert & James Adam and unbuilt Adam, 2001, Volume II, p. 129

Frances Sands, 2012