Bréauté rose to power during the First Barons' War as an unquestioning subject of King John, earning the hate of baronial and monastic leaders alike. He earned the title of John's steward in 1215, a title he kept until the following year. On 28 November 1215, de Bréauté captured Hanslope, Buckinghamshire, a castle of William Mauduit, and he soon after captured Bedford Castle belonging to William de Beauchamp, and in reward was allowed to keep it. In 1216 John divided his army between William Longespée, 3rd Earl of Salisbury and four "alien" captains, one of whom was de Bréauté. When Prince Louis of France invaded in the same year de Bréauté was tasked with holding Oxford against the baronial forces. On 17 July he and the Earl of Chester sacked Worcester, which had allied itself with Louis. In reward John gave de Bréauté the hand of Margaret the daughter of Warin Fitzgerald, the royal chamberlain. She was the widow of Baldwin de Revières (Redvers), former heir to the Earl of Devon, who had died in 1216, and after the death of the 5th Earl in 1217 her son became the 6th Earl. So this marriage made de Bréauté "the equal of an earl" as he was regent for the Earldom until his stepson the 6th Earl reached his majority. As Margaret's dowry he gained control of the Isle of Wight, and as part of her inheritance took Stogursey, also becoming chamberlain to the Exchequer. When John died on 19 October de Bréauté served as the executor of his will, and was one of the royalists who reissued Magna Carta on 12 November 1216.