He was related to many members of the English royal family through his mother, Princess Philippa, Countess of Ulster, who was a granddaughter of King Edward III of England.

However, at the Battle of Bryn Glas on 22 June 1402, Mortimer was defeated, allegedly because some of his Welsh forces defected, and he was taken prisoner.

He forbade the Percys to seek their kinsman's ransom, and by October 1402 began seizing Mortimer's estates, plate and jewels.

However, Hotspur's father, Henry Percy, 1st Earl of Northumberland, was, for reasons never fully explained, slow to move south with his army.

In February 1405, Glyndŵr, Mortimer, and Northumberland entered into the Tripartite Indenture, which proposed a threefold division of the kingdom.

[3] However, after Shrewsbury, Glyndŵr's attacks on the king's forces were largely unsuccessful, and according to T. F. Tout, "Mortimer himself was reduced to great distress".

In the play, Shakespeare accurately identifies him as Hotspur's brother-in-law, but simultaneously conflates him with his nephew by referring to him as "Earl of March".