 Pierrepont was a member of the committee of both kingdoms, and represented the parliamentary party during the deliberations at Uxbridge in 1645; but from that time, according to Clarendon, he forsook his moderate attitude, and contracted more bitterness and sourness than formerly.This statement, however, is perhaps somewhat exaggerated, as Pierrepont favored the resumption of negotiations with the king in 1647, and in the following year his efforts on behalf of peace at Newport, where again he represented the parliamentarians, brought upon him some slight censure from Cromwell.For his services at Newport he was thanked by Parliament; but he retired from active political life soon afterwards, as he disliked the purging of the House of Commons by Colonel Pride and the proceedings against the king.