He was heavily involved in the Second Barons' War, supporting the king and Prince Edward against the rebels led by Simon de Montfort.

[1] William de Valence was the fourth son of Isabella of Angoulême, widow of King John, and her second husband, Hugh X of Lusignan, Count of La Marche, and was thus a half-brother to Henry III, and uncle to Edward I. William was born in the Cistercian abbey in Valence [fr], Couhé-Vérac, Vienne, Poitou-Charentes, near Lusignan,[2] sometime in the late 1220s (his elder sister Alice was born in 1224).

The French conquest of Poitou in 1246 created great difficulties for William's family, and so he and his brothers, Guy de Lusignan and Aymer, accepted Henry III's invitation to come to England in 1247.

In 1304, after William's death, Joan is found vigorously asserting her rights in her lordship of Wexford, appealing directly to the king against a Cort order dispossessing her of her lands there.

He refused to comply with the provisions imposed on the king at Oxford in 1258, and took refuge in Wolvesey Castle at Winchester, where he was besieged and compelled to surrender and leave the country.