Herfast was born in Normandy. He joined William the Conqueror during the invasion of England and after the Battle of Hastings was appointed as head of the royal writing office. In 1069 Herfast became the first government official to be given the title of chancellor.

In 1070 Herfast became the Bishop of East Anglia. He based his activities at Thetford but soon came into conflict with the Abbey of Bury St. Edmund's. Lanfranc of Pavia, the Archbishop of Canterbury, ruled against Herfast and told him that "were you to devote less time to gambling and games of chance, to read the Bible for a change and to learn some canon law... you would cease to argue with your mother church". Herfast died in 1084.