He was the second son of Robert II, Count of Dreux. The latter was in turn the son of Robert I of Dreux, a younger brother of Louis VII of France. Pierre was thus of the Capetian royal house, and a second cousin of Louis VIII of France.
Despite being of royal blood, as the younger son of a cadet branch Pierre's early prospects were that of a minor noble, with a few scattered fiefs in the Ile-de-France and Champagne.
However, in 1212 king Philip II of France needed to find a strong ruler for Brittany. The duchy lay athwart the sea lanes between England and the English territories in Gascony. Furthermore it bordered on Anjou and Normandy, which the English had lost a decade or two before and were eager to recover. It was being ruled with less than a strong hand by Guy of Thouars, as regent for his young daughter Alix. Also worrisome was that Alix's older half-sister Eleanor was in an English prison.
And so king Philip turned to his cousin Pierre, then in his early twenties. Pierre married Alix, and on January 27, 1213 did homage to the king for the Brittany.
There is some ambiguity regarding whether Pierre should be considered duke or count of Brittany. King and Pope (and their courts) always addressed him as "count", but Pierre in his own charters used "duke".
In 1214 king John of England had assembled a formidable coalition against the French. He landed in Poitou while the German king Otto prepared to invade from the north. John chased off some French forces in the north of Poitou, and then moved to the south edge of Brittany, opposite Nantes. Pierre drove him off after a brief skirmish, but did nothing to hinder John's subsequent movement up the Loire valley, where he took a few Breton fortresses and then besieged La Roche-au-Moin. John's Poitivan vassals, however, refused to fight against a French force lead by prince Louis (later Louis VIII of France), and meanwhile the northern invasion was crushed at Bouvines, and the entire invasion foundered.
It is not clear why John attempted to capture Nantes, even less why he would do so the hardest way, via the very well-defended bridge across the Loire. Nor is it clear why Pierre declined to harass his forces from the rear as John marched east. A likely explanation is that the two had come to some sort of agreement, whereby John would leave Brittany alone for the moment, and in return the Bretons would not hinder him elsewhere.
John had a prize he could dangle in front of Pierre: the earldom of Richmond. This great English honor had traditionaly been held by the dukes of Brittany, and in fact a constant theme in Pierre's political affairs was the desire to hold and retain the English revenues from Richmond.
Pierre did not yield to king John's offers to accept the earldom and take up the king's side in his conflicts with the English barons, probably because he deemed the king's prospects too uncertain. Moreover, Prince Louis was again fighting against the English. But when Louis was defeated, Pierre was sent as one of the negotiators for a peace treaty. After the negotiations were complete (in 1218), William Marshal, the regent for the young Henry III of England, recognized Pierre as Earl of Richmond. The center of the earldom's properties in Yorkshire were in the hands of the earl of Chester, whom the regent could not afford to antagonize, but Pierre did receive the properties of the Earldom outside of Yorkshire, which in fact generated the bulk of the earldom's income.
While the negotiations were slowly proceeding, Pierre turned his attention to his next goal. The authority of the dukes of Brittany had traditionally been weak, in comparison to the great peers of northern France. For example, the duke could not limit the building of castles by his counts. Nor did he have the right to guardianship of minor heirs of his vassals. Pierre aimed to re-establish his relationship with his vassals (or subjects) more along the lines of what he knew from the Capetian royal court.
To that end Peter simply declared new rules by fiat, and then faced the inevitable turmoil that resulted from the reaction of his barons. There followed a series of small civil wars and political maneuverings, but by 1223, the barons had all acquiesced to the changes or been dispossessed.
The six Breton bishops were the other threat to the ducal power, for they had substantial landholdings (including control of all or part of the few cities in Brittany), and were recalcitrant in the face of Pierre's attempts to raise revenues by increasing taxes or simply taking possession of episcopal holdings. For this he was excommunicated for a time in 1219-1221. Pierre submitted in the end, but this was not to be the last of his conflict with the bishops.
Pierre's wife Duchess Alix died on October 21, 1221, leaving behind three young children. She was then only 21, and little is known about her beyond the basic genealogical facts.
Her death meant that Pierre was no longer Duke, although he continued to rule the duchy with undiminished authority, as regent for his son Jean, then a boy of four or so.
Alix's death changed Pierre's goals in two ways. First, he aimed to acquire some additional territory, not part of the duchy, to augment his retirement after his son came of age. Second, there was a strong tradition in France that a minor heir should, when coming of age, have his property in the state it was in when he inherited it. Thus Pierre could now some risks without fear of harming the prospects of his son.