Robert was born between 1040–1050, the eldest son of Roger de Beaumont by his wife Adeline of Meulan, a daughter of Waleran III, Count de Meulan, and was an older brother of Henry de Beaumont, 1st Earl of Warwick Robert de Beaumont was one of only a small number of men known to have fought at the Battle of Hastings in 1066, as well as being a cousin of William, and was leader of the infantry on the right wing of the Norman army, as evidenced in the following near contemporary account by William of Poitiers:
'A certain Norman, Robert, son of Roger of Beaumont, being nephew and heir to Henry, Count of Meulan, through Henry's sister Adeline, found himself that day in battle for the first time. He was as yet but a young man and he performed feats of valour worthy of perpetual remembrance. At the head of a troop which he commanded on the right wing he attacked with the utmost bravery and success".
His service earned him the grant of more than 91 English manors confiscated from the defeated English, as listed in the Domesday Book of 1086. When his mother died in 1081, Robert inherited the title of Count of Meulan in Normandy, and the title Viscount Ivry and Lord of Norton. He paid homage to King Philip I of France for these estates and sat as a French Peer in the Parliament held at Poissy. He and his brother Henry were members of the Royal hunting party in the New Forest in Hampshire when King William IIRufus was shot dead accidentally by an arrow on 2 August 1100. He pledged allegiance to William II's brother, King Henry I, who created him Earl of Leicester in 1107. On the death of William Rufus, William, Count of Évreux and Ralph de Conches made an incursion into Robert's Norman estates, on the pretence they had suffered injury through some advice that Robert had given to the king; their raid was successful and they collected a vast booty. During the English phase of the Investiture Controversy, he was excommunicated by Pope Paschal II on 26 March 1105 for advising King Henry to continue selecting the bishops of his realm in opposition to the canons of the church. Sometime in 1106, Henry succeeded in having Anselm, the exiled archbishop of Canterbury, revoke this excommunication. Anselm's act was ultimately ratified by Paschal. According to Henry of Huntingdon, Robert died of shame after "a certain earl carried off the lady he had espoused, either by some intrigue or by force and stratagem." He was the last surviving Norman nobleman to have fought in the Battle of Hastings. Robert de Beaumont was buried at the Abbey of Saint-Pierre de Préaux in Normandy.
