William Douglas, 3rd Earl of March and Ruglen, later 4th Duke of Queensberry (1725-1810), succeeded his father, the 2nd Earl of March, in 1731; his mother, the Countess of Ruglen, in 1748; and his cousin, the 3rd Duke of Queensberry, in 1778. In 1748 he became a member of the Society of Dilettanti, and established himself in London, attending the House of Lords regularly, and becoming a man of fashion, an expert horseman, a successful gambler and ladies' man, and a lover of the opera.

The 3rd Earl also served as a Lord of the Bedchamber in 1761-89; he was a Scottish representative peer from 1761-1810; Vice-Admiral of Scotland in 1767-76; First Lord of the Police in 1776-82; and Lord Lieutenant of Dumfriesshire in 1794. Besides this he was made a Knight of the Thistle in 1763; an English peer as Baron Douglas of Amesbury in 1786; and in the 1790s he was a great supporter of aristocratic refugees of the French Revolution. He died unmarried, leaving the majority of his vast fortune to his illegitimate daughter, Maria Emily Seymour-Conway, Marchioness of Hertford.

15 Piccadilly was built for the 3rd Earl of March in c1760-64 to designs by Matthew Brettingham (1699-1769), and in 1774 Robert Adam was commissioned to make designs for the interior decorative scheme of the first drawing room. The surviving drawings show the unexecuted designs for the ceiling and frieze. It is not known why these designs were not executed. The house (now numbered 138-139) was rebuilt in the nineteenth century to designs by Ralph Selden Wornum (1847-1910).

Literature:
A.T. Bolton, The architecture of Robert and James Adam, 1922, Volume II, index pp. 45, 80; J. Ingamells, A dictionary of British and Irish travellers in Italy: 1701-1800, 1997, p. 640; D. King, The complete works of Robert & James Adam and unbuilt Adam, 2001, Volume II, p. 179; S. Bradley, and N. Pevsner, The buildings of England: London 6: Westminster, 2003, pp. 556-57; Oxford dictionary of national biography online: 'Douglas, William, fourth duke of Queensberry (1725-1810)'

Frances Sands, 2013