William was the son of Simon de Beauchamp and his wife Isabel.
He joined the rebellious barons in 1215 at the beginning of the First Barons' War,[2] entertaining them at his seat of Bedford Castle;[3] as such, de Beauchamp was one of the rebels excommunicated by Pope Innocent III.
[1] William was captured at the Battle of Lincoln on 20 May 1217 but made his peace with Henry III's regency government.
When Breauté fell from power Bedford Castle was besieged and partially destroyed on royal orders, but William was granted licence to build a residence within its Bailey.
He was part of a royal expedition ambushed by Richard Marshal in 1233, and was appointed a Baron of the Exchequer in 1234 and 1237.