Mortimer was dubbed knight by Henry III in person, when that king was celebrating his Whitsuntide court of 1253 at Winchester (Tewkesbury Annals in Ann. Mon. i. 152). In August of the same year he accompanied the king to Gascony (Dugdale, Baronage, i. 141). He was much occupied during the next few years in withstanding the rising power of his kinsman, Llywelyn ab Gruflydd [q. v.], prince of Wales, who, however, in 1256 succeeded in depriving him of his Welsh lordship of Gwrthrynion (Annales Cambria, p. 91; Brut y Tywysogiori). In January 1257 Mortimer had letters of protection while engaged in the king's service in Wales. In April 1258 King Henry promised him large financial aid to enable him to continue his struggle with Llywelyn. Next year his wife's share of the Braose estates was finally determined. On 11 June 1259 Mortimer was among the commissioners assigned to treat for peace with Llywelyn. On 25 June he joined in signing a truce for a year with the Welsh prince at Montgomery (Fœdera, i. 387). But on 17 July 1260 the Welsh attacked and captured Builth Castle, which Mortimer held as representative of Edward, the king's son. Edward did not altogether acquit him of blame (ib. i. 398; Brut y Tywysoffion, s.s. 1259, here unduly minimises Llywelyn's success). But in August Mortimer was again appointed as negotiator of a truce with Llywelyn, though his name does not appear among the signatories of the truce signed on 22 Aug. (Eyton, Shropshire, iv. 217-19).
On the outbreak of the great struggle between Henry III and the barons in 1258 Mortimer at first arrayed himself on the baronial side. He was one of the twelve chosen by the barons to form with twelve nominees of the king a great council to reform the state. He was also appointed one of the permanent council of fifteen who were jointly to exercise the royal power. He was also one of the twenty-four commissioners chosen on behalf of the whole community to treat of the aid which the king required to carry on the Welsh war. Yet the occupation of Mortimer in Wales must have prevented him from taking a very active part in affairs at Westminster, though in the provisions of 1259 he was appointed with Philip Basset to be always with the justiciar (Ann. Burton. in Ann. Mon. i. 479). Moreover, the increasingly close relations between his great enemy, Llywelyn of Wales, and the party of Montfort, must have made it extremely difficult for Mortimer to remain long on the side of the barons. He had close connections with Richard of Clare, seventh earl of Gloucester,
and lord of Glamorgan [q. v.], and with the Lord Edward, who, as holding the king's lands in Wales, was directly associated in interest with the marcher party, of which Mortimer was in a sense the head. But the quarrel of Gloucester and Montfort, and the ultimate breaking off of all ties between Edward and the Montfort party, must have relaxed the strongest ties that bound Mortimer to the party of opposition. In November 1261 the barons were forced to make a compromise with Henry, who on 7 Dec. formally pardoned some of his chief opponents. The names of Leicester and Mortimer were both included in this list ; but what with Leicester was but a temporary device to gain time marks with Mortimer a definite change of policy. Henceforth Mortimer was always on the royal side. All the marcher lords emulated his example, and became the strongest of royalist partisans. The Tewkesbury chronicler makes the hatred felt by the barons for Edward and Mortimer the mainspring of the civil troubles that now again broke out (Ann. Tewkesbury in Ann. Mon. i. 179).
In June 1262 Mortimer was waging war against Llywelyn, who bitterly complained to the king of his violation of the truce (Fœdera, i. 420), and obtained the appointment of a commission to investigate his complaints. But Llywelyn soon took the law into his own hands. In November the Welsh tenants of Mortimer in Melenydd rose in revolt, and called on Llywelyn, who in December attacked Mortimer's three castles of Knucklas, Bleddva, and Cevnllys (Worcester Annals, p. 447 ; Fœdera, i. 423). All three castles were soon taken. Mortimer himself defended Cevnllys, but was forced to march out with all his followers, and Llywelyn did not venture to assail him (ib. i. 423). However, Roger soon recovered this castle (Royal Letters, ii. 229). On 18 Feb. 1263 Mortimer, with other border barons, received royal letters of protection to last until 24 June, or as long as the war should endure in Wales. They were renewed in November of the same year. He remained in Wales, and inflicted terrible slaughter on his Welsh enemies. But he could not undo his rival's successes. His Brecon tenants took oaths to Llywelyn, and next year his castle of Radnor also fell into the hands of the Welsh prince's partisans. Some conquests made by Edward were, however, put into his hands (Rishanger, De Bello, p. 20, Camden Soc.) His English enemies took advantage of his troubles with the Welsh to assail his English estates. The same December that witnessed the loss of the castles of Melenydd saw a fierce attack on his lands by John Gifiard [q. v.] (Tewkes-
