For the next two years he was sent on diplomatic missions by Henry III; however his foreign service appears to have ended in 1257, by which time he was a member of the royal council, had received an appointment in the Welsh Marches,[9][4] and was serving as High Sheriff of Staffordshire[10] and Shropshire.

[citation needed] In April 1258 he and Simon de Montfort were among the seven magnates who revolted against Henry III (the Second Barons' War).

He and the Earl were also among the twelve magnates who drew up plans for reform (the Provisions of Oxford), and were on the council of fifteen members[11] set up to govern England in the King's name.

[4] On 12 April 1261, shortly before his death, Pope Alexander IV issued a papal bull which absolved Henry III of the oath he had taken to keep the Provisions of Oxford; when the King made this known in June of that year, Peter de Montfort was one of three arbitrators elected by the barons to negotiate with the King on their behalf.

[13] According to Cokayne, Montfort 'was now beginning to associate himself definitely with the baronial party', and as a result, the Sheriff of Warwickshire was ordered to prevent him from fortifying Beaudesert Castle.

[13] Simon de Montfort had left England after Henry III's return to power, but was back in the country in April 1263.

[4] On 2 April 1264 he had a safe conduct to Brackley to meet with Henry III's envoys; however on the following day he and his two sons, Peter and Robert, were at Northampton Castle when the Keeper surrendered it to Simon de Montfort the Younger.

On 11 September he was appointed one of the envoys who negotiated the reformation of the English government in the presence of Louis IX of France and the Papal Legate.

[17] On 28 June 1267 he was pardoned by Henry III for 'all trespasses at the time of the disturbance in the kingdom',[17] and eventually recovered part of his father's lands.