Mansell was made Commander-in-Chief of the Parliamentary forces in Glamorgan, under Lord Fairfax, on 17 November 1645. Also in 1645, he was High Sheriff of Glamorgan and was made one of the County Committee of Glamorgan. He was added to the High Court of Justice on 25 June 1651. Mansell was one of the six Members appointed by Cromwell and his officers to represent Wales in the Barebone's Parliament from 4 July to 12 December 1653 On 27 June 1653 -- together with Colonel James Phillips -- he was assigned by the Council of State the official lodgings lately occupied by Sir Harry Vane. He was appointed a Militia Commissioner for South Wales on 14 March 1654 and a Justice of the Peace for Glamorgan in 1655. He was an Assessment Commissioner for raising money there for the State in 1656, and a Commissioner under the Act for ejecting insufficient Ministers and Schoolmasters. He was made a Commissioner for providing for the safety of the Protector on 4 May 1658. On 13 July 1659 he was commissioned to command the Militia Troop in counties Pembroke, Carmarthen and Cardigan and on 30 July following he was appointed to command the whole militia forces in South Wales, horse and foot, "to lead them against the enemy if need be." This was as a result of Sir George Booth's "Cheshire Rising". On 19 September 1659 Mansel wrote to Samuel Moyer, Chairman of the London Committee of Compounding "By the care of our small force in South Wales, it was so kept from insurrection that there will be little work for Sequestration Commissioners. Yet some will be found, for divers delinquents now on hand have estates there and discoveries may be made, of some that went from these parts to the enemy in Chester." In 1660 Mansell was elected Member of Parliament for Cardiff in the Convention Parliament. He was High Sheriff of Glamorgan again in 1677. He was elected MP for Glamorgan in 1679 until January 1681. He was elected MP for Cardiff again from 16 to 28 March 1681. In 1689 he was elected MP for Glamorgan again and sat until his death in 1699.
Thomas Mansel, eldest son and heir, whose white marble mural monument survives in Westminster Abbey, displaying the arms of Mansel: Argent, a chevron between three maunches sable a crescent for difference, with the arms of Games, inscribed in Latin to the effect:
Death and burial
Mansell died at the age of 75, and was buried at Briton Ferry.