Before 14 May 1260, Isabella married her first husband,
John III FitzAlan, the son and heir of
John II FitzAlan, baron of Clun and Oswestry and
de jure earl of Arundel, and Maud de Verdun. In due course John the younger would succeed to the baronies of Clun and Oswestry, but as long as the dowager countess of Arundel remained alive the FitzAlans did not possess the complete earldom or its title. Consequently, Isabella Mortimer never held the title of countess of Arundel, which remained until 1282 with Isabella de Warenne, the childless widow of Hugh d'Aubigny (d.1243). Together Isabella Mortimer and John III FitzAlan had at least two children: •
Richard Fitzalan, Earl of Arundel (3 February 1267- 9 March 1302), married
Alice of Saluzzo, by whom he had issue. • Maud Fitzalan (died after October 1298), married Sir Philip Burnell of Condover, Holgate, Acton Burnell, and Little Rissington, by whom she had issue. Isabella's husband died, aged around 25, in the spring of 1272, leaving underage children. The couple's son Richard FitzAlan was a boy of about five at the time of his father's death and was committed to the wardship of his maternal grandfather, Roger Mortimer of Wigmore. Serious errors in our nineteenth-century sources have led to the enduring misconception that Isabella Mortimer married Ralph d' Ardern soon after the death of John FitzAlan and had three husbands in all. In fact, she remained a widow for over twelve years and was a prominent and active contributor to Marcher society during that time. On 2 September 1285, in a private ceremony at Poling,
Sussex, she married, as his second wife, Robert de Hastang. She was initially heavily penalized for having married without royal licence, before it was discovered that her late father had purchased her marriage rights many years earlier from
King Henry III, and that she and her new husband had absolved themselves of any wrongdoing under the terms of this agreement, by paying a fine to the executors of father's will. She does not appear to have had any children from her second marriage. == Widowhood and the Welsh Frontier ==