 He seems to have been accepted as the spokesman for North Wales in many of the administrative committees that took over the country after the overthrow of the Monarchy, being twice elected to the Council of State, and also serving on the Committee of Both Kingdoms from 1648.However, he was not a member of the smaller council established after Cromwell assumed the Protectorate in 1653.In 1656 Trevor was elected MP for Arundel in the Second Protectorate Parliament, and was one of those advocating the offer of the Crown to Cromwell (to whom he was related by his son's marriage to John Hampden's daughter, Ruth).