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.
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.