Geoffrey, from his name, was apparently from Montbrai, Manche, in the arrondissement of Saint-Lô in the Basse-Normandie region of the former Duchy of Normandy.
Having returned to England he took a leading role in suppressing the wave of English rebellions which erupted in the late summer of 1069.
While William marched north against the uprisings in Mercia and Northumbria, Geoffrey gathered troops from the forces occupying London, Winchester and Salisbury and led them to victory against the rebels besieging Montacute Castle in September 1069.
The bishop, who attended the Conqueror's funeral, joined in the rising against William Rufus in 1088, making Bristol, with which (as Domesday shows) he was closely connected and where he had built a strong castle, his base of operations.
[7] Orderic Vitalis wrote that de Montbray took pride in his noble birth and was better known for his military rather than his clerical ability; he knew more about marshalling soldiers in battle than teaching psalm-singing clerks.
[8] He was an integral part of the close-knit aristocratic group that formed the majority of the Norman episcopate during the reign of Duke William.