 Thomas Bryan Martin was born in Kent, England, in 1731 and was the son of Denny Martin (1695–1762) of Salts Manor and his wife Frances Fairfax Martin (1703–1791).His father and mother married in Loose, Kent.Martin's mother was the daughter of Thomas Fairfax, 5th Lord Fairfax of Cameron (1657–1710) and his wife Catherine Colepeper Fairfax, and thus a sister of Thomas Fairfax, 6th Lord Fairfax of Cameron (1693–1781) and Robert Fairfax, 7th Lord Fairfax of Cameron (1707–1793).