He was the eldest son of William de Warenne, 2nd Earl of Surrey (d1138) by his wife Elizabeth de Vermandois. He was a great-grandson of King Henry I of France, and half-brother to Robert de Beaumont, 2nd Earl of Leicester, Waleran IV de Beaumont, Count of Meulan, and Hugh de Beaumont, 1st Earl of Bedford.
He participated in the Second Crusade. Still in his minority in 1137 he was serving with Stephen, King of England in Normandy being one of those young nobles who initially fled the battle. Stephen pursued them, held them and did his best to pacify them but did not make them fight.
At Easter 1138 he accompanied his half-brother Waleran de Beaumont, 1st Earl of Worcester on an embassy to Paris for the purpose of ratifying a treaty between the English and French kings.
He was killed at the Battle of Mount Cadmus while the crusader army was marching across Anatolia on their way to the Holy Land. In December 1147 the French-Norman force reached Ephesus.
They were joined by remnants of the army of the Holy Roman Empire, which had previously suffered heavy losses at the Battle of Dorylaeum in 1147. They marched across southwest Turkey and fought an unsuccessful battle at Laodicea (3–4 January 1148) on the border between the Byzantine Empire and the Sultanate ofRum.
On 6 January 1148 they battled again in the area of Mount Cadmus, where Turks ambushed the infantry and non-combatants only, because they had become separated from the rest of the army.
King Louis VII and his bodyguard of Knights Templars and noblemen recklessly charged the Turks. Most of the knights were killed, including William, and Louis barely escaped with his life. His army arrived later at the coastal city of Adalia.
The battle is recorded by Odo of Deuil, personal chaplain to Louis, in his narrative De Profectione.
