Remarks:
BIOGRAPHICAL: He is remembered for having been converted to the rebel cause at the time of the Welsh revolt led by Owain Glyn Dwr - and for dying in defence of Harlech Castle.
He was the second son of the 3rd Earl of March, also called Edmund, and brother of Roger, who became the 4th Earl. The senior Edmund had married Philippa, a daughter of the second son of Edward III, and it was through this line that some held the Earls of March had a better right to the throne of England than the descendants of Edward's fourth son, John of Gaunt. John's son became Henry IV.
To complicate matters even more, the 5th Earl of March, born in 1391 and succeeding to the title in 1398, was yet another family member named Edmund. When Henry Bolingbroke displaced Richard II to take the crown as Henry IV in 1399, the Earl of March was only seven years old and his uncle Edmund - the one under discussion here - accepted Bolingbroke as his king.
The new king faced border warfare with Scotland, where the Earl of Northumberland and his son, Henry Percy, known as Hotspur, defended Henry's realm. There were also problems in Wales, where Owain Glyn Dwr had been declared prince in September 1400. The Welsh uprising was opposed on Henry's behalf by the Lords of the Marches, Edmund Mortimer among them. The connection between the defenders of the borders was close, for Edmund's sister had married Hotspur.
By 1402 Owain's successes had become intolerable, and learning that the Welshman was at Pilleth, west of Ludlow, Edmund moved to attack him. On 22 June 1402 he fell on the Welsh force below the hill of Bryn Glas, and he appeared to be winning the day when the Welsh archers among his own forces turned their fire on the English, and hidden reinforcements rushed to Owain's aid, overwhelming their attackers.
Edmund and his second-in-command, Lord Grey, were captured, but King Henry agreed to ransom only Grey. Edmund thus remained in Owain's power and before long, though in which order we are not told, had allied himself with the Welshman and married his daughter, Katherine. Before long these two men had made further alliance with Hotspur.
The young Percy had gathered an army to meet another threat from the Scots, but that came to nothing and the force was turned against Henry. The king and his son, Prince Hal, fought Hotspur at Shrewsbury on 21 July 1403. Mortimer was meant to join his brother-in-law but never showed up. Hotspur was defeated and killed.
Hotspur's father, Northumberland, took his son's place in the plotting against Henry but was soon beaten, and the English from then on made steady gains in Wales. By 1409 only the castles of Aberystwyth and Harlech remained to Glyn Dwr, and both were taken by the English that year. It was in the defence of Harlech that Edmund Mortimer died. By contrast his nephew, the young Edmund, enjoyed good relations with both Henry IV and Henry V. He became Lieutenant of Ireland and died there, of the plague, in 1425.
In 1398 when young Edmund, the 5th earl, nephew of Sir Edmund, succeeded to the title while still a minor, Sir Edmund became the most powerful representative of his family. He supported the usurpation of the throne by the Lancastrian Henry IV in 1399. In 1402, however, Mortimer was captured by the rebellious Welshman Owen Glendower and when the suspicious king forbade his ransom, Edmund entered an alliance with Glendower and married his daughter. Supporting the claim of his young nephew to the throne, he and Glendower continued to fight even after the defeat of their allies, the Percy family. However, Glendower began to suffer defeats, Mortimer's own effectiveness declined, and he died when besieged by royal forces at Harlech. Some sources say he possibly died of the Plague.