Isabel of York, the only daughter of Richard of Conisburgh, 3rd Earl of Cambridge, and Anne de Mortimer, was born about 1409.
[1] Through her father, she was the granddaughter of King Edward III's fourth surviving son, Edmund of Langley, 1st Duke of York, and his first wife, Isabella of Castile.
Through her mother, Anne de Mortimer, Isabel was the granddaughter of Roger Mortimer, 4th Earl of March, and Lady Alianore Holland (granddaughter of Lady Joan of Kent, Princess of Wales) and the great-great-granddaughter of Edward III's second surviving son, Lionel of Antwerp.
In 1412, at three years of age, Isabel was betrothed to Sir Thomas Grey (1404 – d. before 1426),[b] son and heir of Sir Thomas Grey (c. 1385 – 1415) of Heaton in Norham, Northumberland, and his wife, Alice Neville, the daughter of Ralph Neville, 1st Earl of Westmorland.
The elder Sir Thomas Grey was an associate of Isabel's father who also lost his life in the Southampton Plot.