In 1198 Cantilupe was steward to John, Count of Mortain, the future King John, in which year his uncle Fulk de Cantilupe was also a member of John's household. From 1200 to 1204 he served as Sheriff of Worcestershire and in 1204 as Under-Sheriff of Herefordshire. In 1205 he took part in the ineffectual expedition to Poitou. In 1207, he was Sheriff of Worcestershire, serving until the end of the John's reign in 1216. In 1209, following his appointment as Sheriff of Warwickshire and Sheriff of Leicestershire, his main residence became Kenilworth Castle in Warwickshire. Cantilupe was granted several manors formerly held by rebel barons during 1215–16, at the time of the signing of Magna Carta in 1215. He was commissioned by John to negotiate the return of such rebels to peaceable relations. He served as gaoler of baronial hostages, which action probably gained him the description by the contemporary chronicler Roger of Wendover as one of John's "evil counsellors". In 1204, Cantilupe was granted the Warwickshire manor of Aston, to which as was usual for the purpose of differentiation, was appended his family name, now Aston Cantlow. This manor had previously been held by William de Tankerville "the Chamberlain" before it escheated to the crown. In 1205 Cantilupe was granted the manor of Eaton, in Bedfordshire, which became the caput of the Cantilupe feudal barony, where he built a castle described by the monks of nearby Dunstable Priory in the Annals of Dunstable as being "a serious danger to Dunstable and the neighbourhood". The grant was for knight-service of one knight and was in exchange for the manor of Great Coxwell, Berkshire, which had been granted to him previously but the grant was deemed compromised. Eaton had been held at the time of William the Conqueror by the latter's uterine half-brother Odo, Bishop of Bayeux, but later escheated to the crown.
Following the death of King John in 1216, many of the royal appointees to governorships of royal castles were reluctant to hand over their castles to the regency council which governed during the minority of his son, Henry III. They believed themselves obliged to hold their castles until Henry should have achieved 14 years of age, when he would be able to follow his own policy. These many refusals met with a forceful response from the council. In 1217, under the regency council, during which year he was a Baron of the Exchequer, Cantilupe was at the siege of Mountsorrel Castle, Leicestershire, which was razed to the ground, and was also at the Second Battle of Lincoln. He served the council at the siege of Bedford in 1224. He later served in Wales and Brittany.
Marriage and children
He married Mazilia de Braci, daughter and heiress of Adulf de Braci of Mentmore in Buckinghamshire, who brought him that manor and others in Kent, and by whom he had issue including:
William II de Cantilupe, eldest son and heir, who following in his father's footsteps served as steward of the household to King Henry III, son of King John;
