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". Reginald's sister was Rohesia, wife of Henry de Pomeroy (died c.1135), feudal baron of Berry Pomeroy in Devon.
Reginald was invested with the Earldom of Cornwall by his first cousin (by blood) King Stephen (1135-1154), son of Henry I's sister Adela, Countess of Blois. He later supported Stephen's enemy and claimant of the English throne, the Empress Matilda, only legitimate daughter of Henry I, and thus by order of Stephen he forfeited his lands and honours. In about 1173 he granted a charter to his free burgesses of Truro in Cornwall, and addressed his meetings at Truro to "All men both Cornish and English" suggesting a differentiation of nations. He served as High Sheriff of Devon from 1173-1174.
He married Mabel FitzRichard, daughter of William FitzRichard, a substantial landholder in Cornwall, by whom he had the following progeny:
He also had illegitimate children by his mistress Beatrice de Vaux (also known as de Valle), later the wife of William Brewer: