He was one of the nine godfathers of Prince Edward, the future King Edward I of England. He served as Sheriff of Kent for 1239–40. In 1258 after returning from a pilgrimage to the Holy Land, Humphrey fell away, like his father, from the royal cause to that of the barons. He served as a nominee of the opposition on the "committee of twenty-four" which was appointed in the Oxford Parliament of that year, to create the Provisions of Oxford to reform the administration. The alliance of Simon de Montfort with Llywelyn ap Gruffudd of North Wales brought Bohun back to royal allegiance. He headed the first secession of the Welsh Marchers from the party of the opposition (1263), and was amongst the captives whom the Montfortians took at the Battle of Lewes in 1264. He was amongst the victors at the Battle of Evesham in 1265, which extinguished the power of de Montfort, at which however his eldest son Humphrey V de Bohun was mortally wounded. Humphrey was selected as one of the twelve arbitrators to draw up the Dictum of Kenilworth (1266), by which the disinherited rebels were allowed to make their peace.