The illegitimate son of Robert I, Duke of Normandy, he succeeded to the dukedom as a child in 1035. His early life was fraught with danger—three of his closest advisers were murdered and an attempt was made on his own life. Twice he faced major rebellions. In 1047 he was saved only by the intervention of the French king, Henry I, who helped him in battle. Between 1053 and 1054 Henry failed to seize Normandy for himself. William’s claim to the English throne was based on the promise allegedly given to him in 1051 by Edward the Confessor. With papal backing he landed in England and defeated Harold II at the Battle of Hastings (1066). While the English leaders considered their next move William laid waste parts of Sussex, Surrey, and Hertfordshire. He was crowned on Christmas Day at Westminster Abbey.

The period 1067 to 1071 was characterized by a number of rebellions against his rule in Northumbria, Wessex, Mercia, and the Isle of Ely. His suppression of them was ruthlessly effective. Much of his later life he spent in Normandy fighting against the French king Philip I but before his death he initiated the Domesday Book (1086). He died after being wounded on the battlefield, fighting Philip I of France.