Her maternal grandparents were William IV of Ponthieu and Alys, Countess of the Vexin, daughter of Louis VII of France and Constance of Castile.

In 1235, Queen Blanche invoked that promise on behalf of her minor son, King Louis IX, and threatened to deprive Simon of all his lands if Joan married Henry III.

Henry therefore abandoned the project for his marriage to Joan and in January 1236 married instead Louis IX's sister-in-law Eleanor of Provence.

Berengaria determined to find Ferdinand another wife, and her sister Blanche suggested Joan of Dammartin, whose marriage to the king of Castile would keep her inheritance from falling into hostile hands.

Sometime in 1253, she became the ally and supporter of another of her stepsons, Henry of Castile, who also felt Alfonso had not allowed him all the wealth their father had meant him to have.

This further strained her relations with Alfonso and in 1254, shortly before her daughter Eleanor was to marry Edward of England, Joan and her eldest son Ferdinand left Castile and returned to her native Ponthieu.

During her marriage to Jean de Nesle, Joan ran up considerable debts and also appears to have allowed her rights as countess in Ponthieu to weaken.

By using English wealth, Edward and Eleanor restored stability to the administration and the finances of Ponthieu, and added considerably to the comital estate by purchasing large amounts of land there.