Joan married
William I, Count of Hainaut, on 23 May 1305. They had been engaged in 1302, and the marriage took place to ensure William's loyalty toward France. The chronicle
Van den derden Eduwaert by
Jan van Boendale describe Joan as a good and pious woman who wisely fulfilled the traditional task of mediator. She fulfilled the traditional role to supervise the household and court and act as a benefactor of artists, but also kept herself updated on political affairs and maintained contact with her family in both France and England, which kept her well informed of political matters. During the many absences of her spouse, Joan was left to handle the economy and political affairs of the realms, receive diplomats and travel between Hainaut and Holland to maintain control. She was a supporter of her cousin,
Isabella of France, Queen of England, in her struggle against her husband, King
Edward II of England. In December 1325, Joan traveled to France to attend the funeral of her father and had talks with Queen Isabella and her brother, King
Charles IV of France. This brought about an alliance between Hainaut, the
Queen of England, and the English exiles, who were in opposition to the English king and his favorite,
Hugh Despenser the Younger. Isabella's son,
Edward of Windsor, became engaged to Joan's daughter,
Philippa of Hainault, and Isabella raised an army in their lands. It was also from there that Isabella and her lover,
Roger Mortimer, 1st Earl of March, began their invasion of England. In 1332, after Joan's daughter Philippa had become queen, Joan arranged a wedding between Isabella's daughter
Eleanor of Woodstock and
Reginald II, Duke of Guelders, and she visited her daughter Philippa in England. == Later life: mediator ==