He was the son of Lord William Fleming, 5th Earl of Wigtown (d.1681) and Henrietta Seton, dau of Charles Seton, 2nd Earl of Dunfermline.
Lord John Fleming married 1stly., (1698) Margaret Lindsay, dau of Colin Lindsay, 3rd Earl of Balcarres; and 2ndly. Mary Keith (d 1721), dau of William Keith, 9th Earl Marischal, by whom he had a daughter Lady Clementina Fleming.
His grandfather, John, 3rd Earl, (s. in 1650) was a Royalist and fought for Montrose and both his uncle John, 4th Earl, and his father William, 5th Earl, maintained their family’s ancestral loyalties.
Lord Fleming and his brother Charles who would succeed him as 7th Earl, supported the Jacobites in 1715.
After the latter’s death without sons he was succeeded in the Fleming estates, but not in the title, by his niece Lady Clementina Fleming (1719 - 1799) who married Charles Elphinstone, 10th Lord (1711 - 1781).
F. 10748 – Early Armorial
Arms: Fleming of Biggar quartering Fraser of Oliver Castle
On the controversial claim to the title which became dormant after the death without issue of Charles, the 7th Earl in 1747 see, William Anderson, Genealogy and Surnames: With Some Heraldic and Biographical Notices ..., Ritchie, 1865, p. 89
