Roger Mortimer, born 11 April 1374 at Usk in Monmouthshire, was the eldest son of Edmund Mortimer, 3rd Earl of March, by his wife Philippa Plantagenet, who as the daughter of Lionel of Antwerp, 1st Duke of Clarence, and granddaughter of King Edward III, had a claim to the crown which she passed on to her children. He had a younger brother, Edmund Mortimer, and two sisters, Elizabeth, who married Henry 'Hotspur' Percy, and Philippa (1375-1401), who married firstly John Hastings, 3rd Earl of Pembroke (d.1389), killed in a tournament at Woodstock Palace, secondly Richard de Arundel, 11th Earl of Arundel (1346-1397), beheaded in 1397, and thirdly, Sir Thomas Poynings.