Isabella was born in about 1263 in Warwickshire, England. She was the only daughter of William de Beauchamp, 9th Earl of Warwick and Maud FitzJohn. She had a brother, Guy de Beauchamp, 10th Earl of Warwick who married Alice de Toeni, by whom he had seven children. Her paternal grandparents were William de Beauchamp of Elmley Castle and Isabel Maudit, and her maternal grandparents were Sir John FitzGeoffrey, Lord of Shere, and Isabel Bigod.