He was the son of Walter de Bereford, with the family name coming from the village of Barford, Warwickshire.
In 1287 his brother, Osbert de Bereford, a previous High Sheriff of Warwickshire and Leicestershire, bought a property in Wishaw, and after his death a few years later the land was left to William.
In the 1280s he also married Margaret, daughter of Hugh de Plessy, who brought lands in Wittenham, Berkshire, with whom he had at least one child, Edmund Bereford, who later became a King's Clerk.
Bereford was associated with Piers Gaveston, a favorite of Edward II, perhaps because he had bought land in the Honour of Wallingford held by the earl.
[2] Standing against the barons seems not to have harmed his career; in 1318 after attempts to reform the king's household he was among the ministers retained in office, where he stayed until his death in 1326.