Mortimer, Sir Edmund de, 1376–1409, English nobleman; youngest son of Edmund de Mortimer, 3d earl of March. 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 (see Percy, Sir Henry and Northumberland, Henry Percy, 1st earl of). However, Glendower began to suffer defeats, Mortimer's own effectiveness declined, and he died when besieged by royal forces at Harlech.