In his youth, he was reported courageous and skillful in military exercises. He was, however, also prone to a laziness and weakness of character that discontented nobles and the King of France exploited to stir discord with his father William.
In 1077, he instigated his first insurrection against his father as the result of a prank played by his younger brothers William Rufus and Henry, who had poured water through the floor into Robert's chambers. As a result of the insult, Robert attempted to seize the castle of Rouen and afterwards spent several years wandering in aimless fighting before being reconciled with his father.
In 1087, he father died, having divided the Norman dominions between his two eldest sons. To Robert, he granted the Duchy of Normandy and to William Rufus he granted the Kingdom of England. Of the two sons, Robert was considered to be much the weaker and was generally preferred by the nobles who held lands on both sides of the English Channel, since they could more easily circumvent his authority. At the time of their father's death, the two brothers made an agreement to be each other's heir.
Robert took as his close advisor Ranulf Flambard, who had been previously a close advisor to this father.
In 1096, Robert left for the Holy Land on the First Crusade. At the time of his departure he was reportedly so poor that he often had to stay in bed for lack of clothes. In order to raise money for the crusade, he mortaged his duchy to his brother William for the sum of 10,000 marks.
In 1100, during Robert's absence, William Rufus died, allowing their younger brother Henry to seize the crown of England with popular support. Upon his return, Robert, urged by Flambard, lead an invasion of England to retake the crown from his brother Henry.
In 1101, Robert landed at Portsmouth with his army, but his lack of popular support among the English allowed Henry resist the invasion. Robert was forced by diplomacy to renounce his claim to the English throne in the Treaty of Alton.
In 1105, however, Robert's continually stirring of discord with his brother in England prompted Henry to invade Normandy.
In 1106, Henry defeated Robert's army decisively at the Battle of Tinchebray and claimed Normandy as a possession of the English crown, a situation that endured for over a century. Captured after the battle, Robert was imprisoned for the rest of his life.
His is buried in the abbey church of St. Peter in Gloucester, where an elaborate sepulchre was later built.