William Montagu, born at Cassington, Oxfordshire in 1301, was the second but eldest surviving son of William Montagu, 2nd Baron Montagu, and Elizabeth de Montfort, daughter of Sir Peter Montfort of Beaudesert, Warwickshire. by Matilda/Maud de la Mare daughter and heiress of Henry de la Mare of Ashtead, Surrey, Royal Justice, Seneschal of William Longspree II Earl of Salisbury. The Montagu family, a West Country family with roots going back to the Conquest, held extensive lands in Somerset, Dorset and Devon. The father, William Montagu, distinguished himself in the Scottish Wars during the reign of Edward I, and served as steward of Edward II's household. Some members of the nobility, including Thomas of Lancaster, viewed Montagu with suspicion, as a member of a court party with undue influence on the king. For this reason he was sent to Aquitaine, to serve as seneschal. Here he died on 18 October 1319. Even though he sat in parliament as a baron, the second lord Montagu never rose above a level of purely regional importance.