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 his father's death in 1138, William became the third Earl of Surrey. 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. On 2 February 1141 he and his half-brother Waleran were again with King Stephen at the Battle of Lincoln but fled at the initial charge of the enemy forces. They both joined Queen Matilda but on King Stephen's release they were once again among his followers, and William witnessed a royal charter at Canterbury in late 1141.
Charters
With his brother Ralph, William was a joint donor in numerous charters issued by his parents and was a witness to his father's charter, all to Longueville Priory near Rouen, Normandy. He was also a donor with his brother and both parents to the priory of Bellencombre in 1135. William was also the primary donor in a number of charters after his father died, between 1138 and 1147.One of particular instance occurred during a livery of seisin in 1147. At that event, William gave a large gift to the Lewes Priory which was secured with a lock of hair from his own and from his brother Ralph's head cut by Henry of Blois, bishop of Winchester, before the altar of the priory church. Lewes Priory had been founded by William's grandparents, William de Warenne, 1st Earl of Surrey, and his wife Gundrada, probably in 1081.
Marriage and children
William married Adela, daughter of Count William III of Ponthieu, by his wife Helie daughter of Odo I, Duke of Burgundy. They had one child and sole-heiress, a daughter, Isabel de Warenne, suo jure 4th Countess of Surrey. She married firstly William of Blois, the second son of king Stephen, who became Earl Warenne or Earl of Surrey. After he died childless in October 1159, she remarried to Hamelin, half-brother of King Henry II, who became Earl Warenne or Earl of Surrey. He adopted the surname "de Warenne", and the earldom continued in his descendants.
Death on crusade
He was one of the nobles who, along with his second cousin, King Louis VII of France, took crusading vows at the Council of Vézelay in 1146, and he accompanied the initial army of the Second Crusade the next year. William 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 Laodiceaon the border between the Byzantine Empire and the Sultanate of Rum. 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.
