 He was the son of Edward Courtenay, 1st Earl of Devon by his wife Elizabeth Courtenay, daughter of Sir Philip Courtenay (b.1445) of Molland, 2nd son of Sir Philip Courtenay (18 January 1404 – 16 December 1463) of Powderham by Elizabeth Hungerford, daughter of Walter Hungerford, 1st Baron Hungerford (d. 1449).William's parents were thus distant cousins, sharing a common descent from Hugh Courtenay, 10th Earl of Devon (1303–1377).