Seigneur (Lord) of Beaumont, Pont-Audemer, Brionne, and Vatteville. These were critically important castles in Normandy. From 1081 he was Comte (Count) of Meulan also, which an inheritance through his mother. 
In England he became the first Anglo-Norman Earl of Leicester (not counting any earls before 1066). His very significant English lands were centred there and in neighbouring Warwickshire.
He was at the battle of Hastings. Keats-Rohan, explains the original medieval source: "According to William of Poitiers, his first engagement as a knight was on the field of Hastings, where he performed outstandingly well."
"On 5 June 1118 Count Robert I of Meulan died at one of his English residences. His end was not a happy one: life at King Henry's court tended to put a burden on the consciences of its inmates."
"The count's corpse was carried back to Normandy and buried in the chapter house of his family's foundation of St Peter of Préaux, near his father, Roger de Beaumont, and his uncle, Robert fitz Humphrey."
He married in 1096 to Isabel (also called Elizabeth) de Vermandois, daughter of Hugh de Crépi (called "the Great"), Count of Vermandois. Orderic Vitalis wrote that this was his second wife, but Complete Peerage thinks his account is unlikely to be correct, as the lady named, Godechilde daughter of Ralph de Toeni, married when Robert was still, and was still quite young at this marriage.
The issue of Robert de Meulent by Isabel was a daughter named Emma, born, according to Orderic, in 1102; two sons (twins), baptised Waleran and Robert, born in 1104; a third son, known as Hugh the Poor, afterwards Earl of Bedford, and three other daughters, Adeline, Amicia, and Albreda, all of whom must have been born after 1104, when their father, then Earl of Leicester, was well stricken in years. Orderic, indeed, says he had five daughters, the fifth being named Isabel, after her mother. [What old source was this copied from?]
Emma de Beaumont (b.1102)
Waleran IV de Beaumont, Count of Meulan, 1st Earl of Worcester (b. 1104), eldest twin and heir.
Robert de Beaumont, 2nd Earl of Leicester & Earl of Hereford (b. 1104), twin
Hugh de Beaumont, 1st Earl of Bedford (b. circa 1106)
Adeline de Beaumont
m.1 Hugh IV of Montfort-sur-Risle
m.2 Richard de Granville of Bideford (d.1147)
Aubree/Alberada de Beaumont m. Hugh II of Châteauneuf-Thimerais.
Agnes de Beaumont (nun)
Maud/Matilda de Beaumont m. William Lovel. (c.1102)
It may be possible to confirm family relationships with Robert by comparing test results with other carriers of his Y-chromosome or his mother's mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree:
We need a source for the daughter Havoise (has profile) and indeed Agnes (who has no profile but is mentioned). Also the paragraph about when the children were born is clearly from an old book, but which?
If the birth year is only a guess perhaps we should set it a bit earlier, because apparently he was at Hastings in 1066 (although remarked as being a young man). See for example Wikipedia's article on the Companions of William.