He is one of the few proven companions of William the Conqueror known to have fought at the Battle of Hastings in 1066. Tradition says he gave up the role of standard bearer, his hereditary office, to Walter Giffard, in order to be able to fight closer to William, duke of Normandy.
He married Isabel de Montfort, daughter of Simon I de Montfort. There was a feud with Guillaume d'Evreux and his wife Helvise de Nevers, recorded by Ordericus Vitalis. This came to open war in 1091–92, when Guillaume attacked Conches. A settlement was reached. They later co-operated in attacking Robert de Beaumont, 1st Earl of Leicester's county of Meulan.
He had widely spread holdings, as recorded in the Domesday Survey. His seat was at Flamstead in Hertfordshire. He held Clifford Castle. It is also believed that he held assets in the village of Hose, Leicestershire, which was split into two manors, Tosny's and that of the title holder of the Norman Belvoir Castle.
Robert de Stafford was his brother.