Thomas Lucas was born before his father's marriage with Elizabeth Leighton. His father purchased for him the manor of Lexden, Essex, from the heirs of Robert Ratcliffe, earl of Sussex
Lucas obtained the command of an English troop in the Dutch service, and was knighted by Charles I on 14 April 1628.In December 1638 Wentworth, Earl of Strafford gave him the command of a troop in the Irish army. He was one of the officers in whom Ormonde most confided during the Irish Rebellion of 1641, and held the rank of commissary-general of the horse. He distinguished himself at the Battle of Kilrush (15 April 1642), and was badly wounded at the Battle of New Ross on 18 March 1643.
From 1642 Lucas was a member of the Irish Privy Council, took part in negotiating the cessation of hostilities in 1643 and the treaty of 1646, and was consequently held a delinquent by parliament. He was, however, allowed to compound for his estate on paying a fine of £637 in 1648, and died before October 1649.