Firstly, on 30 March 1231 at Fawley, Buckinghamshire, to Isabel Marshal, daughter of William Marshal, 1st Earl of Pembroke (by his wife Isabel FitzGilbert) and widow of Gilbert de Clare, 5th Earl of Gloucester. Isabel died in child-birth at Berkhamsted Castle on 17 January 1240, and was buried at Beaulieu Abbey. By Isabel he had three sons and a daughter:[7]
John of Cornwall (31 January 1232 – 22 September 1232), born and died at Marlow, Buckinghamshire, buried at Reading Abbey.
Isabel of Cornwall (c. 9 September 1233 – 6 October 1234), born and died at Marlow, Buckinghamshire, buried at Reading Abbey.
Henry of Cornwall (2 November 1235 – 13 March 1271) Henry of Almain, murdered by his cousins Guy and Simon de Montfort, buried at Hailes Abbey.
Nicholas of Cornwall (b. & d. 17 January 1240 at Berkhamsted Castle), died shortly after birth, buried at Beaulieu Abbey with his mother.