 Reginald was born in Dénestanville in the Duchy of Normandy, an illegitimate son of King Henry I (1100–1135) by his mistress Sybilla Corbet, a daughter and co-heiress of Sir Robert Corbet, lord of the manor of Alcester, Warwickshire, who was at some time the wife of "Herbert the King's Chamberlain".During the war between Matilda and Stephen, Reginald, who supported Matilda, was in control of Cornwall.Subsequently forced out of Cornwall by Stephen's forces, Reginald lost the earldom to Alan of Richmond.