William de Valence, 1st Earl of Wexford and 1st Earl of Pembroke, born Guillaume de Lusignan or de Valence (1225-1230 – May 16 or 18, 1296) was a French nobleman and Knight, who became important in English politics due to his relationship to Henry III. He was heavily involved in the Second Barons' War, supporting the King and Prince Edward against the rebels lead by Simon de Montfort. He took the name de Valence ("of Valence").
He was the fourth son of Isabella of Angoulême, widow of king John of England, and her second husband, Hugh X of Lusignan, Count of La Marche, and was thus a half-brother to Henry III of England, and uncle to Edward I. William was born at Valence, near Lusignan, sometime in the mid-to-late 1220s (his elder sister, Alice was born 1224, and two elder brothers followed her).
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. The king found important positions for all of them; William was soon married to a great heiress, Joan de Munchensi or Munchensy (c. 1230 – after September 20, 1307), Lady of Swanscombe and Countess of Pembroke, the only surviving child of Warin de Munchensi, Lord of Swanscombe and Earl of Pembroke, and wife Joan Marshal, daughter and eventual co-heiress of William Marshal, 1st Earl of Pembroke. Her portion of the Marshal estates included the castle and lordship of Pembroke and the lordship erected earldom of Wexford in Ireland. The custody of Joan's property was entrusted to her husband, along with, apparently, the title of Earl of Pembroke and Earl of Wexford between 1250 and 1260.
However, in 1259 William and de Montfort were formally reconciled in Paris, and in 1261 Valence was again in England and once more enjoying the royal favour. He fought for Henry at the disastrous Battle of Lewes, and after the defeat again fled to France, while de Montfort ruled England. However, by 1265 he was back, landing in Pembrokeshire, and taking part in the Siege of Gloucester and the final royalist victory at Evesham. After the battle he was restored to his estates and accompanied Prince Edward, afterwards Edward I, to Palestine. From his base in Pembrokeshire he was a mainstay of the English fights against the Welsh princes, assisting in the conquest of North Wales. He also went several times to France on public business and he was one of Edward's representatives in the famous suit over the succession to the crown of Scotland in 1291 and 1292.
William and Joan de Munchensi (described above) had the following children: