In 1660 he was appointed as one of twelve commissioners sent from Tyrone to treat with
Charles II. He was
knighted, and was appointed to the post of
Prime Serjeant, the most senior law office in Ireland. However
Ormonde, the
Lord Lieutenant, had always distrusted him and preferred to take advice only from the
Attorney General for Ireland, Sir
William Domville, so that in a few years Mervyn's role as Crown legal adviser effectively lapsed. From then on the Attorney General of Ireland was always regarded as the senior Law Officer. He was chosen Speaker of the House of Commons in May 1661 when again member for Tyrone, against the wishes of the King, who would have preferred William Domville. It was said that his fellow MPs chose him for his eloquence "of a style much admired at the time", full of Scriptural allusions and quotations from the Classics, mixed up with quotations from recent legal writers like Sir
Edward Coke. Shortly thereafter he went to England for nine months between September 1661 and May 1662 to take part in negotiations on the
Act of Settlement 1662. When he returned he played an influential role in the House and was at the same time involved in the Court of Claims set up under the Act of Settlement to adjudicate land claims. This led to charges of
corruption against him. He was Speaker until the dissolution of Parliament in 1666, although he had greatly offended the King, and his loyalty to the Crown was deeply suspect. He continued with his legal practice, but his later years are rather obscure. In 1663 he was chosen as
Recorder of Carrickfergus, (the choice was by popular vote and was not a Crown appointment), but never took up office. ==Family ==