Thomas, who was a member of the great family of Hamilton, being a son of ThomasHamilton of Priestfield, was a lawyer who became a lord of session as Lord Drumcairn in 1592.
The earl's eldest son Thomas, the 2nd earl (1600-1640), was a covenanter and a soldier, being killed by an explosion at Dunglass castle on the 30th of August 1640.
Thomas was a supporter of George I. during the rising of 1715, and was a representative peer for Scotland from 1716 to 1734.