She was the eldest daughter of Roger Mortimer, 1st Earl of March, the de facto ruler of England from 1327 to 1330, and his wife Joan de Geneville, Baroness Geneville.

Her maternal grandparents were Piers de Geneville, of Trim Castle and Ludlow, and Jeanne of Lusignan.

From Roger's point of view, the marriage was meant to secure an earlier alliance with an important lord of the Welsh Marshes.

[3] Their eldest son Maurice married Elizabeth le Despenser, despite the fact that it had been his grandfather Roger Mortimer that was largely responsible for the execution of Elizabeth's father Hugh le Despenser in 1326.

Her marriage to Berkeley was confirmed, and her offspring declared legitimate by Pope John XXII in 1329.