Immediately after his own coronation, Robert's father Hugh began to push for the coronation of Robert. "The essential means by which the early Capetians were seen to have kept the throne in their family was through the association of the eldest surviving son in the royalty during the father's lifetime," Andrew W. Lewis has observed, in tracing the phenomenon in this line of kings who lacked dynastic legitimacy. Hugh's claimed reason was that he was planning an expedition against the Moorish armies harassing Borrel II of Barcelona, an invasion which never occurred, and that the stability of the country necessitated a co-king, should he die while on expedition. Ralph Glaber, however, attributes Hugh's request to his old age and inability to control the nobility. Modern scholarship has largely imputed to Hugh the motive of establishing a dynasty against the claims of electoral power on the part of the aristocracy, but this is not the typical view of contemporaries and even some modern scholars have been less sceptical of Hugh's "plan" to campaign in Spain. Robert was eventually crowned on 25 December 987. A measure of Hugh's success is that when Hugh died in 996, Robert continued to reign without any succession dispute, but during his long reign actual royal power dissipated into the hands of the great territorial magnates.

Robert had begun to take on active royal duties with his father in the early 990s. In 991, he helped his father prevent the French bishops from trekking to Mousson in the Kingdom of Germany for a synod called by Pope John XV, with whom Hugh was then in disagreement.

As early as 989, having been rebuffed in his search for a Byzantine princess, Hugh Capet arranged for Robert to marry the recently widowed daughter of Berengar II of Italy, Rozala, who took the name of Susannah upon becoming Queen. She was many years his senior. She was the widow of Arnulf II of Flanders, with whom she had children, the oldest of whom was of age to assume the offices of count of Flanders. Robert divorced her within a year of his father's death. He tried instead to marry Bertha, daughter of Conrad of Burgundy, around the time of his father's death. She was a widow of Odo I of Blois, but was also Robert's cousin. For reasons of consanguinity, Pope Gregory V refused to sanction the marriage, and Robert was excommunicated. After long negotiations with Gregory's successor, Sylvester II, the marriage was annulled.

Finally, in 1001, Robert entered into his final and longest lasting marriage to Constance of Arles, the daughter of William I of Provence. She was an ambitious and scheming woman, who made life miserable for her husband by encouraging her sons to revolt against their father.

Robert, however, despite his marital problems, was a very devout Catholic, hence his sobriquet "the Pious." He was musically inclined, being a composer, chorister, and poet, and making his palace a place of religious seclusion, where he conducted the matins and vespers in his royal robes. However, to contemporaries, Robert's "piety" also resulted from his lack of toleration for heretics: he harshly punished them. Indeed, he is credited with advocating forced conversions of local Jewry, as well as mob violence against Jews who refused. Furthermore, Robert reinstated the Roman imperial custom of burning heretics at the stake.

The kingdom Robert inherited was not large, and in an effort to increase his power, he vigorously pursued his claim to any feudal lands which became vacant, which action usually resulted in war with a counter claimant. In 1003, his invasion of the Duchy of Burgundy was thwarted and it would not be until 1016 that he was finally able to get the support of the Church and be recognized as Duke of Burgundy.

The pious Robert made few friends and many enemies, including his own sons: Hugh Magnus, Henry, and Robert. They turned against their father in a civil war over power and property. Hugh died in revolt in 1025. In a conflict with Henry and the younger Robert, King Robert's army was beaten and he retreated to Beaugency outside Paris, his capital. He died in the middle of the war with his sons on 20 July 1031 at Melun. He was interred with Constance in Saint Denis Basilica. He was succeeded by his son Henry, in both France and Burgundy.

Robert had no children from his short lived marriage to Susanna. His illegal marriage to Bertha gave him one stillborn son in 999, but only Constance gave him surviving children. Robert also left an illegitimate son.