Trevor lived most of his early life in the shadow of his father and uncles, only emerging as a significant figure in his own right in the later 1640s. In 1617 he was appointed to one of the auditorships of the duchy of Lancaster jointly with his uncle Thomas Trevor*, a sinecure he enjoyed for 20 years before surrendering his interest to his cousin Sir Thomas Trevor†, bt.
Although Trevor’s father held a substantial Flintshire estate, the family spent most of their time at their town house in and around London. Thus Trevor’s return to Parliament for Denbighshire in December 1620 can almost certainly be ascribed to the influence of his uncle Sir Richard Trevor†, one of the greatest landowners in the eastern half of the county. At the previous election Sir Richard had supported Henry Salusbury of Lleweni in an unsuccessful contest with Simon Thelwall*, but the 1620 election was apparently settled by consensus, as the return was signed almost exclusively by the Thelwall faction.
Trevor may have stood for re-election to the Denbighshire seat in 1624, but on this occasion Sir Eubule Thelwall carried the day. It was presumably too late to procure a seat elsewhere by the time the news reached London, and Trevor had to wait for an appropriate vacancy, which occurred with the death of the Flintshire MP Sir John Hanmer in June 1624. The family estates at Plas Têg gave Trevor’s father sufficient influence to make a nomination, which was presumably endorsed by Hanmer’s widow, one of Sir Richard Trevor’s daughters. With a new session planned for the New Year, the by-election took place on 6 Dec., when the indenture was signed by a broad cross-section of the county’s major gentry families.
Trevor was almost certainly put forward for re-election in Flintshire in January 1626, although the contest ultimately came to a poll between John Salusbury* of Bachegraig, head of a cadet branch of the Denbighshire family, and Sir Thomas Hanmer†, 2nd bt., 13-year old heir to the 1624 MP. The return of a candidate as young as Hanmer would have been so exceptional that it seems likely he was substituted for Trevor, his uncle, on the morning of the election, to save the latter from the humiliation of a defeat. Whatever Trevor’s last-minute misgivings, his faction was clearly not lacking in support: in addition to their own tenants Trevor and Hanmer could call upon those of John Hanmer, bishop of St. Asaph, and Sir Roger Mostyn*. However, the sheriff apparently played a pivotal role in excluding many of Hanmer’s freeholders during a two-day poll, and while his conduct provoked a formal complaint, the investigation by the Commons’ privileges committee remained incomplete at the dissolution.
Understandably, Trevor chose not to stand for a Welsh constituency at the next election in 1628, when he was returned for Great Bedwyn, a seat customarily controlled by the local magnate, the 3rd earl of Hertford. No personal connection has been found between the two men, and Trevor, who was appointed a gentleman of the Privy Chamber only a few weeks later, was perhaps recommended by the lord chamberlain, the 1st earl of Montgomery (Sir Philip Herbert*).
Trevor inherited Plas Têg upon his father’s death in 1630, and Trevalyn from his uncle Sir Richard Trevor in 1638, while he retained the family house at Oatlands by agreement with his brother Charles. He also acquired his father’s quarter share in the farm of the sea coal duty, an investment which yielded him about £1,500 a year before the Civil War, well worth the £20,000 the farmers advanced towards the king’s campaign against Scotland in 1639 in return for a renewal of their contract.
Trevor was appointed to the executive committee of Both Kingdoms during the Second Civil War, but he absented himself from the Commons for six months after Pride’s Purge. Following his return he served twice on the Council of State, his second term being ended abruptly by the ejection of the Rump on 20 Apr. 1653.