Son of Sir William Douglas (d. 1733), 2nd Bart of Kelhead, and Helen Erskine (d. 1754), a half-sister of Boswell's mother Euphemia. Brother of Charles and William Douglas. Married to Christian Cunningham (1710-1741), daughter of Sir William Cunningham of Caprington. Father of William Douglas, Catherine Douglas and about six other children.
Boswell visited John Douglas and his family at Kelhead on October 6 and 7, 1762. He wrote of Douglas that "by his princely improvements [of the estate] Sir John Douglas has burthened his estate with about £30,000. [...] a lively man, but hurried away by fanciful project."