Roger was the third son of
Roger Mortimer, a powerful
Marcher lord in the Welsh border territories, and
Maud de Braose, Baroness Mortimer who was also an important Marcher landowner in her own right. The family were from the second rank of
parvenu nobility elevated by the king as a reward for fierce loyalty to the
Plantagenet dynasty. But he was said to be a lecherous and violent man. He married Lucy de la Wavre, the daughter of Sir Robert la Wavre, Lord of Hampton Wafer, Herefordshire, by whom he had one legitimate son, also named Roger. Presumably they were married by 8 June 1286, when Roger de Mortimer was presented at the manor of
Tedstone Wafer. In 1269,
Gruffydd II, prince of
Powys Fadog, died, and his lands were divided between his four sons. In 1277, the eldest son,
Madog II, died; his two young brothers, Llywelyn and Owain, were still children, leaving them, and their lands, at the mercy of his eldest brother
Gruffydd Fychan I. Mortimer was appointed by
King Edward Longshanks to be the guardian of Owain and Llywelyn, but four years later their bodies washed up in the River Dee; Mortimer was accused of their murder. Mortimer, guilty or not, was granted their lands - the Cantref of
Swydd y Waun. It is possible that Mortimer had needed the lands to raise his nephew,
Roger, as his guardian. In 1282, the ruling princes of Gwynedd attacked lands in the
Perfeddwlad granted to King Edward under the
Treaty of Aberconwy. Consequently, the Welsh wars broke out, and Mortimer, a professional soldier, was a captain in the victorious royal army. He and his brother,
Edmund Mortimer, 2nd Baron Mortimer of Wigmore, have been implicated in the plot that lured
Llywelyn ap Gruffudd, the last native Welsh prince of Wales, out of Gwynedd to his death with promises of allegiance. At some point, probably after 1295, he began work on
Chirk Castle, possibly designed by
James of St George, the architect of
Beaumaris Castle, but the castle remained unfinished at the time of his death. Mortimer fought at the
Battle of Falkirk (1298), when
William Wallace was finally defeated. On 6 February, he was created Lord of Chirk. The Mortimers supported the King's policy in Scotland and on the Marches. Chirk pledged allegiance to Edward II, and was with the young King when he went to negotiate his marriage with
Isabella of France. They arrived at Dover on 19 January 1308, and Mortimer left Edward to cross the channel. The Mortimers acquired huge estates, taking ruthless control of Welsh strongholds. Roger was also granted the constableships of Blaenyllfori and Dinas in north Wales. Such was his power he was effectively a surrogate prince of Wales. Anticipating the nobles' conflict with Edward's
favourite,
Piers Gaveston, Chirk avoided confrontation when Gaveston was appointed Regent of England in the king's absence abroad. ==Feud with Griffith de la Pole==