Robert is given by some Victorian historians as a son of Adam de Brus, by his spouse Emma de Ramsay.
Cokayne states that the family name is derived from Bruis, now Brix, in the arrondissement of Valognes. Some modern historians contend that the name may have come from Brix, Manche, near Cherbourg in the Cotentin Peninsula, and that they came to England after King Henry I of England's conquest of Normandy (i.e.: at the same time as Alan fitz Flaad, the FitzAlan ancestor of the Stuart Royal Family, notwithstanding that they were Bretons).
What is known clearly is that this Robert de Brus is first mentioned during the period 1094 and 1100, as a witness to a charter of Hugh, Earl of Chester, granting the church of Flamborough, Yorkshire, to Whitby Abbey. Possibly the Earl of Chester about 1100–1104 enfeoffed Robert of certain portions of his Cleveland fee in Lofthouse, Upleatham, Barwick, Ingleby, and other places. Between 1103–1106 Robert de Brus attested with Ralph de Paynel and 16 others a charter of William, Count of Mortain, to the abbey of Marmoutier. In 1109 at a Council of all England held at Nottingham, he attested the charter of King Henry I confirming to the church of Durham certain possessions which the men of Northumberland had claimed. During the period 1109–1114 he appears in early charters in possession of numerous other manors and lands in Yorkshire, and in the same period he attested a charter of Henry I issued at Woodstock, Oxfordshire. He appears in the Lindsey Survey made 1115–1118 in possession of even further lands. There is a strong presumption that the King had given Robert his Yorkshire fee soon after the battle of Tinchebrai (28 September 1106). Robert was present at the great gathering of northern magnates at Durham in 1121, and sometime during the period 1124–1130 he was with the King at Brampton. About 1131 he was in the retinue of Henry I at Lions, in Eure. At about the same time he attested with three of his personal knights a confirmation with Alan de Percy to the monks of Whitby. It is said that Robert had been given some 80 manors in Yorkshire by King Henry. It is evident that Robert kept up his connexions with other Normans too. A member of the Feugeres family, of Feugeres, Calvados, arr.Bayeux, canton of Isigny, witnessed charters of this Robert de Brus circa 1135 in Yorkshire.
The friendship between Robert de Brus and David FitzMalcolm (after 1124 King David I of Scotland), who was present in France with King Henry and was granted much of the Cotentin Peninsula, may have commenced at least as early as 1120, at Henry's Court. When David became king, he settled upon his military companion and friend the Lordship of Annandale, in 1124, There is, however, scant evidence that this Robert ever took up residence on his Scottish estates.
After the death of King Henry, David refused to recognise Henry's successor, King Stephen. Instead, David supported the claim of his niece and Stephen's cousin, Empress Matilda, to the English throne and taking advantage of the chaos in England due to the disputed succession there, he took the chance to realise his son's claim to Northumberland. These actions Robert de Brus of Annandale could not countenance and as a result he and King David parted company, with Robert bitterly renouncing his homage to David before taking the English side at the Battle of the Standard in 1138. Before the battle, Robert had made an impassioned plea to David, calling to his remembrance how he and other Normans had by their influence in Scotland, as far back as 1107, obliged King Alexander to give a part of the Scottish Kingdom to his brother David. The appeal was in vain. Robert, and his eldest son Adam, joined the English army, while his younger son, Robert, with an eye on his Scottish inheritance, fought for David.
Robert is said to have married twice:
(1) Agnes, daughter of Geoffrey Bainard, Sheriff of York and
(2) Agnes, daughter and heiress of Fulk de Paynel of Carleton, North Yorkshire. Farrer mentions both marriages and in particular points out that the superior of Carleton Manor was de Brus, and that Paynel held it of him.
It is unclear by which spouse his sons were but authorities usually give her as Agnes de Paynel.Adam de Brus I, eldest son and heir upon whom devolved, under feudal law, all the English estates. he only survived his father by 12 months, his wife's name not known in the records.
Robert de Brus, the younger son, upon whom his father had settled the Scottish Lordship of Annandale, plus several wheat-producing ploughates at Skelton, Yorkshire, in his lifetime.