William Douglas, 4th Duke of Queensberry, (16 December 1724 – 23 December 1810) was a British peer and courtier. He was popularly known as Old Q (because that letter was painted on his carriage's door) and was reputed as a high-stakes gambler. In 1799 he was estimated the eighth-wealthiest man (or small family unit) in Britain, owning £1M (equivalent to £98,800,000 in 2025). He is one of ten known British millionaires that year, the royal family excluded.
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Family and royal appointee
Born in Peebles, Queensberry was the only son of William Douglas, 2nd Earl of March, and his wife, Lady Anne Hamilton. He was great-grandson of the 1st Duke.
A friend of the Prince of Wales, the future George III, Douglas was appointed Gentleman of the Bedchamber to him when he became king in 1760. He was appointed a Knight of the Thistle in 1761 and was one of the 16 Scottish representative peers for an elected term or possibly more from 1761, and was Vice Admiral of Scotland from 1767 to 1776. However, due to behavior during the king's unusual, long-lasting, mental health latter-life illness he was deprived of his office as Gentleman of the Bedchamber in 1789, and for a while took refuge abroad. Later, he was Lord Lieutenant of Dumfries from 1794 until 1810.
He succeeded his father in the Earldom of March in 1731 and his mother in the Earldom of Ruglen in 1748. He succeeded his cousin Charles as Duke of Queensberry in 1778, and was created Lord Douglas, Baron Douglas, of Amesbury in the County of Wiltshire in the Peerage of Great Britain on 8 August 1786.