According to Cokayne, (p. 525), "John Marshal, the founder of this family, owed his political advancement to his near relationship to William Marshal, Earl of Pembroke, and his feudal positions to his marriage with Aline, eld. da. and coh., and afterwards sole heir, of Hubert IV de Rye, lord of the barony of Rye. John Marshal first appears on the page of history in 1197, as one of the Knights, under command of William Marshal, sent by Richard I to assist the Counts of Flanders and Boulogne against France. In 1203 he was in Normandy with King John, and in the following year went to Ireland in the service of the Earl Marshal, while the Earl went to Normandy in the service of the King. On 12 Nov. 1207 he was made Marshal of Ireland. He accompanied King John on his Irish expedition, 1210. From 1213 to the end of the reign he was constantly occupied in the King's service in England; in 1215 he was with King John at Runnymede, and one of those by whose advice the King gave his assent to Magna Carta; later in the year he was one of the embassy to the court of Rome. Returning, he accompanied the King on his march to the North, and attended him at his death at Newark, 19 Oct. 1216. Nine days later he was present at the Coronation of Henry III at Gloucester, and was a member of the council of his uncle, guardian of the young King. He was among the 'barons' in the Earl Marshal's forces in the relief of Lincoln Castle. From this time till his death he was continually occupied in the public service in England, Ireland, or abroad."
In the footnotes, Cokayne states that "William Marshal's elder brother John (whose heir he was) died in 1194, leaving a widow, da. of Adam de Port. John is described in a charter of King John as nepes Willelmi Marescalli comitis de de Penbroc. The very detailed biography of the Earl Marshal ... makes no mention of a second brother John, and the Earl's nephew appears to have had no patrimony; it is therefore probable that he was an illegitimate son of the Earl's elder brother".