Son of Sir John Douglas, 3rd of Kelhead (and son of Boswell's mother's half-sister Helen Erskine) and Christian Cunninghame (1710-1741). Married Grace Johnstone (1746-1836) in 1772, with whom he had seven children, Mary (b.1773)1, Christian (b. 1774), Catherine-Heron (b. 1775), Charles (b. 1777)2, John (1779-1856)3, Henry-Alexander (b. 1781)4 and William Robert Keith (1783-1859)5
He sat as a Member of Parliament for Dumfries Burghs (1768-1780), a seat which was later held by his youngest son, William. An army captain in 1762.
- 1. Mary Douglas married (1817) Major-General Sir Thomas-Sydney Beckwith (1769-1831).
- 2. Charles Douglas married (1803) Caroline Montagu-Scott, daughter of the 3rd Duke of Buccleuch and Queensberry. On December 23, 1810, he became the 6th Marquis of Queensberry, succeeding his distant relative William Douglas, 4th Duke.
- 3. John Douglas married (1817) Sarah Sholto Douglas. In 1837 he succeeded his older brother as the 7th Marquess of Queensberry.
- 4. Henry Alexander Douglas married (1812) Elizabeth Dalzell
- 5. MWilliam Robert Keith Douglas married (1821) Elizabeth Irvine. He was MP for Dumfries Burghs from 1812 to 1832.
Boswell possibly met William Douglas on October 6, 1762, when he visited Douglas' family at Kelhead. In his journal, Boswell mentions a "Mr. Douglas, son to Sir John, an officer in the Greys, an amiable young fellow whom I hope to see in the circumstances which he deserves". However, Sir John did have two other surviving sons, at least one of whom was in the army. Furthermore, it is doubtful whether Boswell would refer to a 32-year-old as a young fellow.
On December 20, 1762, in London, Boswell dined with Douglas and Captain Maxwell.