Mortimer, Roger (1374–98), 4th earl of March and 7th earl of Ulster , was eldest son of Edmund (qv), 3rd earl of March, and his wife Philippa, daughter of Lionel (qv), duke of Clarence, and became the foremost magnate in Ireland on the death of his father (December 1381). At the age of 7, he was appointed lieutenant of Ireland (January 1382) to replace his father, with his uncle, Sir Thomas Mortimer, to act as his deputy. However, this solution did not work well and he was replaced by Sir Philip Courtenay (qv) in July 1383. He was also an important figure in England because of his extensive estates, and as a potential heir to the throne if Richard II (qv) died childless. The wardship of his lands was granted to Richard fitz Alan, earl of Arundel, but his marriage was granted to the king's half-brother, Thomas Holland, earl of Kent, who arranged a marriage (c.1388) between his daughter Eleanor and March.
March was knighted by the king in April 1390, received livery of his Irish lands in June 1393 and of his English and Welsh lands in February 1394, and acted as ambassador to the Scots to treat for peace (February 1394). Though appointed lieutenant of Ireland in 1392, he did not go to the lordship till Richard II went in October 1394. His main goal was to restore his family's authority in Ulster and Connacht, and he succeeded in forcing Ó Néill (qv) to restore the traditional dues owed to him as earl of Ulster. However, many of the outstanding issues that needed to be settled between him and the Gaelic Irish of Ulster were still left undecided by the king when he left Ireland in May 1395. March remained as lieutenant of Ireland, but his authority was limited to Ulster, Connacht, and Meath (Sir William Scrope (qv) acted as justiciar in Leinster, Munster, and Louth). While in Ireland, he acted as the greatest of the Anglo-Irish magnates, and his activities in Ulster were instrumental in unravelling the arrangements made by Richard II in Ireland. From April 1396 he took an increasingly aggressive stance towards the Gaelic Irish of Ulster, which was expressed in raids against the O'Farrells and O'Reillys in Longford and Cavan, and culminated in an attack on Ó Néill, with the 5th earl of Kildare (qv) and the 3rd earl of Ormond (qv) supporting him. His position in Ulster depended on military might, and weakened steadily in 1396. March returned to England (January–June 1397), and attempted to steer a neutral path among the factions there. He was appointed sole lieutenant of Ireland in March 1397 and was sent back to Ireland with less money and fewer troops, indicating that he was to use his own wealth to support his office. Basing himself in Dublin, he achieved a precarious supremacy over the Gaelic lords in Leinster before returning to England (January 1398); while there, he came under increasing suspicion by the king. He returned to Ireland in April 1398 but the king sent Thomas Holland, duke of Surrey, to replace him almost immediately. However, by the time Surrey arrived in Ireland, March had died of wounds sustained in an attack on the O'Byrnes on 20 July 1398. His body was returned to England for burial at Wigmore abbey, and once again the lands and titles of the earldom of March passed to a minor, erasing any gains that had been made in the previous decade.