In 1256, she founded the nunnery of Longchamp in part of the Forest of Rouvray (now called the Bois de Boulogne), west of Paris.

While pursuing the traditional feminine interests such as embroidery, she took special pleasure in working on priestly vestments.

By virtue of the Treaty of Vendôme in March 1227, Isabelle was betrothed to Hugh, eldest son and heir of Hugh X of Lusignan, with the marriage contract being signed on June 1230;[4] however, she refused to celebrate the formal wedding due to her fixed determination to remain a virgin, although she never became a nun.

[5] By the papal bull of 26 May 1254, Pope Innocent IV allowed her to retain some Franciscan friars as her special confessors.

Pope Alexander IV gave his sanction on 2 February 1259 to the new Rule, which was composed especially for this monastery by Isabelle along with a team of Franciscan university masters including Bonaventure.

In the Rule the nuns were called the Sisters of the Humble Order of Servants of the Most Blessed Virgin Mary.

She refused to become abbess, which allowed her to retain her wealth and resources, so she could support her abbey and continue to give to the poor.

After nine days her body was exhumed; according to the religious legend, it showed no signs of decay, and many miracles were said to have been wrought at her grave.