Trevor became at once prominent in the service of Richard II, acting as his ambassador in Scotland in 1397, and just before the king's abdication in 1399 was rewarded with high secular office in Wales. But he was one of the first to desert the ill-fated monarch, who was actually made captive in Trevor's own diocese, and it was the bishop himself who read the sentence of deposition in full Parliament. He continued prominent as a royal emissary and spokesman during the early years of the Lancastrian regime, and as late as 1403, long after the Glyn Dŵr revolt had started, he is found deputising in Wales for prince Henry, afterwards Henry V. But suddenly towards the end of the following year he transferred his allegiance to Glyn Dŵr.

Though the facts of his career suggest that he was a typical self-seeking churchman of his age, it is to his credit that before he adhered to the patriot cause he had protested unavailingly in Parliament against high-handed treatment of his fellow-countrymen. His diplomatic experience, moreover (the Northumberland alliance of 1405 and the crucial decisions of the Pennal Assembly in 1406 owed much to his mediation and counsel), and his legal knowledge (he was a doctor in laws) were of the highest value to Glyn Dŵr, a fact which appears to have been recognized by the other side when, in 1409, the bishop is associated by name with Owain as one of the king's arch-enemies. He d. on 10 or 11 April 1410, while on a mission to Paris, and was buried there, in the infirmary chapel of the abbey of S. Victor. He had at least one admirer (as bishop of S. Asaph during the years before the rebellion) for Iolo Goch composed a highly eulogistic poem (possibly two) in his honour.

There are strong grounds for concluding that Trevor was also the author of a well-known work on heraldry — the Tractatus de Armis, as well as of its Welsh version — and that he translated the life of S. Martin (Buchedd Sant Marthin) into Welsh (see B.B.C.S., iv, 3, 4; v, 1). E. J. Jones has also suggested that the authorship of several historical works of the period may be attributed to him (see Speculum, xii, 196 et seq.; xv, 464 et seq.).