He was the son of William the Conqueror's mother Herleva, and Herluin, viscount of Conteville. Odo, Bishop of Bayeux was his older full brother. He was probably born around 1040, but perhaps a few years earlier.
Around 1055 Duke William was consolidating his hold on the duchy, and having disposessed the count of Mortain, gave the county to Robert. Mortain was a frontier territory, bordering on Brittany and Maine, and Robert contributed to the defense of the duchy by constructing castles at the town of Mortain, and at St. Hilaire-du-Harcouet, le Tilleul, and Tinchebrai.
During the next decade Robert was a close counselor of his half-brother, appearing frequently at his court, including the councils at which the invasion of England was planned. He surely fought at the Battle of Hastings itself, though he chronicles of the time are not specific, and more importantly he contributed a significant part of the invasion fleet.
After the conquest, Robert obtained a large holding in England, including the strategic Rape of Pevensey in Sussex, and other lands guarding London, including Berkhamsted, Lambeth and Bermondsey. In 1072 he was entrusted with Cornwall after the previous ruler returned home to Brittany, though probably he never formally became earl. Nevertheless he was the second greatest lay magnate in England.
Robert seems to have been uninterested in English politics -- some say he lacked the cleverness for it -- and he spent most of the remainder of the Conqueror's reign in Normandy. He stayed loyal to William to the end, even when his brother Odo of Bayeux rebelled and was imprisoned. At the dying duke's bedside he successfully argued for Odo's release.
Odo seems to have pushed him into action, for Robert took part in the 1088 English baronial rebellion which attempted to replace William Rufus with Robert Curthose. After Pevensey surrendered and the rebellion failed, Rufus pardoned Robert, while Odo was exiled to Normandy. Their differing treatment may reflect contemporary assessment of Robert's lack of initiative.
He married Matilda, daughter of Robert II of Montgomery, and was succeeded by their son William, Count of Mortain.
Robert Golding, "Robert of Mortain", Anglo-Norman Studies XXIIIReferences