D'Aubigny died on 7 May 1243, by which time her father and uncles had died, leaving her as a teenage widow with a small fortune. She retained the title "Countess of Arundel", despite the Arundel estates being distributed amongst D'Aubigny's sisters.[1] On 29 May 1243 a proposal for her second marriage was granted to a favourite of the King, the Savoyard Pierre de Genevre, but she did not remarry. Patent Rolls show that she had provision to remain unmarried if she chose but would have had to make a payment to Pierre to be released.[2]
Isabel became a religious patron and in 1249 she founded Marham Abbey in Norfolk on part of her land, the only female English convent that was part of the Cistercian order at its founding.[3] In 1252, she visited the Cistercian Abbey at Waverley where she consulted with the abbot and granted 4 marks and a cask of wine to the monks.[2]
In 1252, upon the death of Thomas Ingoldsthorpe, King Henry III took possession of his lands. However Thomas was a tenant of de Warenne's, and owed her a quarter of a knight's fee, so she requested the value from the King. Henry refused and so she complained about this behaviour to his face, suggesting that Magna Carta was being ignored. She then left Henry's court, without obtaining permission.[3] Henry would pay back the debt a year later, and forgive the fine that she had been charged in appealing the case,[1] providing that 'she says nothing opprobrious to us as she did when we were at Westminster'.[2]
Isabel died before 23 November 1282 and was buried at Abbey.[2]