1st Earl of Surrey. He was one of the leaders at the battle of Mortemer. After the battle of Arques, in February, 1054, William the Conqueror's force was large enough for it to be divided into two contingents operating to the west and east of the Seine. The duke himself, with men from middle Normandy, faced the invaders who were advancing under the French king, Henri I, through the Evrecin. On the other side of the river, Robert, count of Eu, with Hugh de Gournay, Walter Giffard, Roger de Mortimer, and the young William de Warenne, came out from their own lands to withstand the eastern incursion of the French force under count Odo and count Rainald, which seems to have been unprepared for this levy from
eastern Normandy. Having entered the duchy by way of Neufchatel-en-Bray, the French army advanced to the neighborhood of Mortemer, and there gave itself up to unrestrained rape and pillage. Widely scattered and demoralized, it thus offered itself as an easy target. The slaughter was considerable. The battle was of decisive importance. The Norman victory was complete, and the King of France withdrew. The battle of Mortemer was a major crisis in Norman history, and never again was duke William to be faced by so formidable a threat to his power.
At some time in or after 1054, duke William gave William the castle of Mortemer, which had been forfeited by his kinsman, Roger de Mortimer, after the battle of Mortemer. William also acquired lands at Bellencombre, the castle of which became the caput of the Warenne barony in Normandy. In 1066 he was one of the Norman barons summoned by duke William to a council on hearing that Harold had been crowned king after the death of Edward the Confessor. He took part in the invasion of England and was present at the battle of Hastings. He was rewarded with lands in 13 counties. In 1067 he was one of the Norman nobles whom the Conqueror left in England to support his vice-regents, William FitzOsbern and the bishop of Bayeux. In 1075 he was one of the two chief justiciars who were in charge of England when the earls of Hereford and Norflok rebelled, and who summoned them to the KingÕs court, and on their refusal crushed the rebellion. To secure his loyalty he was created earl of Surrey shortly after Easter, April 16, 1088. About 1078-1082, he founded Lewes Priory in Sussex as a cell of Cluny Abbey. He was mortally wounded at the siege of Pevensey before the end of May, 1088, and died at Lewes in June.
A possible alternative parentage for William is given in the account of Ralph Mortimer I in the Dictionary of National Biography : ". . .Roger (de Mortimer, Ralph's father). . .was accordingly deprived of his castle of Mortemer, which was transferred to his nephew, William de Warren, son of his brother Ralph, and afterwards first Earl of Surrey. However, it has recently been suggested that William de Warenne was son of Ralph (Ranulf), husband of Emma, and that Ralph, along with Roger de Mortimer, were in turn sons of another Ralph, the husband of Beatrice (with Beatrice the descendant of Gunnor's clan). At least as explained in the argument, the dual (actually triple, counting William's brother) Ralphs seem to better fit the available data. The discussion of the connection of Beatrice to Gunnor's family is confused and confusing in this source, which at one point shows Beatrice's supposed brother as the husband of Gunnora's niece (which would negate a supposed descent), while later showing the father's wife as the niece. In any event, William de Warenne belonged to a kinship group descended from the siblings of Gunnor that maintained a sense of collective kinship well through 1100.