In 1048 William of Normandy had a dispute with William of Warling. William began to doubt the loyalty of William of Warling and gave his land in Mortain to Robert.
When William the Conqueror decided to invade England in 1066, he invited his three half-brothers, Robert, Richard Fitz Gilbert and Odo of Bayeux to join him. One Norman chronicler claims that Robert of Mortain contributed 120 ships to William's invasion fleet.
After his coronation in 1066, William the Conqueror claimed that all the land in England now belonged to him. William retained about a fifth of this land for his own use. The rest was distributed to those men who had helped him defeat Harold at the Battle of Hastings.
Robert of Mortain was granted manors in Cornwall (248), Yorkshire (196), Northamptonshire (99), Devon (75), Sussex (54), Dorset (49) and Buckinghamshire (29). He also had manors in ten other counties. His 793 manors made him the second largest landowner in England.
In exchange for this land. Robert had to promise to provide the king with sixty knights. In order to supply these knights, barons divided their land up into smaller units called manors. These manors were then passed on to men who promised to serve as knights when the king needed them.
Robert Curthose, William's eldest son, was expected to become king of England when his father died. However, William preferred William Rufus, who, unlike Robert, had remained loyal to his father. William Rufus became king of England in 1087 and the following year, Robert, Geoffrey of Coutances, Odo of Bayeux and Richard Fitz Gilbert led a rebellion against his rule. Many Normans remained loyal and William Rusfus defeated the rebels.
Robert of Mortain died in 1091.