Fought as a knight at the battle of Mortemer, 1054. Married Gundrada of Flanders. Granted Mortemer Castle by Duke William. Fought at Hastings, 1066. Received great grants of lands and built castles at Lewes in Sussex, Reigate in Surrey, and Castle Acre in Norfolk. Granted Conisborough, Yorkshire, 1069. Fought against the refugees in Ely, 1071. Joint chief justiciar, 1075. Helped to suppress the rebellion of the Earls of Hereford and Norfolk. Founded the Cluniac priories of St. Pancras, Lewes, 1077, and Castle Acre. Fought in Main, 1085. Remained faitful to Rufus, 1088. Granted the earldom of Surrey, abt. 1088. Fatally wounded at the siege of Pevensey Castle (The concise dictionary of national biography. Part 1 : from the beginnings till 1900. London : Oxford University Press, 1969, pg. 1368).

Buried Chapter House, Lewes, Sussex (WWW site : Directory of royal genealogical data).

The family derived its name from the fief of Varenne in St.-Aubin-le-Cauf, arrondissment of Dieppe. William, count of Warren (Varenne) in Normandy, was descended from Gautier de St-Martin and a neice of the duchess Gonnor, who had issue, I, Raoul de Warren, a benefactor of the abbey of Trinitie du Mont in the middle of the 11th century, was the father of William de Warren I and of Roger de Mortemer, father of Raoul de Mortemer, who was present at Hastings; 2, sire de St-Martin, possibly named Gautier, ancestor of the family of this name in Normandy and England. Orderic Vital styles William the cousin or kinsman (consanguineo ejus) of Roger de Mortemer; however, this is an error. Norman People published this pedigree, Gautier de St-Martin, and a neice of the aforesaid duchess had a son, William de St-Martin, whose issue were: I, Roger de Mortemer, father of Raoul de Mortemer, a warrior at Hastings, 2, Raoul de Warren, and 3, side de St-Martin, but this makes too many generations for the known facts (vide addenda, 183). William de Warren is first mentioned in history in connection with the battle of Mortemer in 1054 by Orderic Vital, and again as having attended the council at Lillebonne, where it was determined to invade England. He later was one of the powerful seigniors who attended duke William to the conquest, and Wace (l. 13586) records "De Garenes i vint Willeme," but nothing of importance is chronicled concerning him at Hastings. In 1067 he was one of the nobles entrusted with the government of England during the king's absence in Normandy under the jurisdiction of bishop Odo and William Fitz Osberne. In 1074 he was associated with Richard de Bienfaite in the suppression of the rebellion of the earls of Herefod and Norfolk and as a joint-justice-general with him for administering justice throughout the whole realm. His reward was princely, since he held the great baronies of Castle Acre in Norfolk, Lewes in Sussex, where he usually resided, and Coningsburg in Yorkshire, with twenty-eight towns and hamlets in its soke. In all he possessed 300 manors and was created first earl of Surrey by William Rufus. The reason for this enormous reward was probably because he married Gundreda, who is believed to have been the daughter of queen Matilda; she died in 1085. This theory is supported by a charter of William de Warren to Lewes priory, in which he states that his donations, among others, were for queen Matilda, the mother of his wife. It is conjectured that Grundreda and Gherbod the Fleming, created earl of Chester, her brother, were the children of queen Matilda by a former marriage, probably clandestine, and therefore not reported by the historians of the day. William de Warren I was succeeded by his son, WIlliam II, earl of Warren and Surrey, who married Elizabeth, daughter of the great earl of Vermandois, the widowed countess of Meulent, by whom he had, among other children, William III, the last earl of his line, who succeeded him and died in the Holy Land, leaving an only child Isabel, who inherited his vast domain and through whom the famimly descended. In addition to Wace, William de Warren is reported at Hastings by William of Poiters, Orderic Vital and Benoit de St-More (Crispin, M. Jackson. Falaise roll : recording prominent companions of William, Duke of Normandy at the conquest of England. Baltimore : Genealogical Publishing Company, 1969, p. 52-53). The above would seem to inidcate a birthdate sometime before 1035. Note also Raoul de Warren is listed as William's father, conflicting with the information from the www site "Directory of royal genealogical data."