Afonso IV married his daughter, Maria, to Alfonso XI of Castile, but quickly learned that she was being mistreated by her husband. Alfonso's cousin, Juan Manuel, Prince of Villena, had also been rebuffed by the king when his daughter Constanza was rejected in favor of the Portuguese princess. Feeling as though his daughter was being dishonored, Afonso was glad to enter into an alliance with Juan Manuel and married his son and heir, Peter, to Constance.

When Constanza arrived in Portugal, Inês de Castro, the daughter of an aristocratic Castilian land owner, accompanied her as her lady - in - waiting. Peter fell in love with Inês very quickly, and the two conducted an affair that lasted until Constance's death in 1345. The scandal of this affair caused Afonso to banish Inês from court, but this did not end the relationship, and the two began living together in secret.

According to the chronicle of Fernão Lopes, during this period, Peter began giving Inês's brothers important positions at court. This behavior alarmed Afonso and made him believe that upon his death, the Portuguese throne would fall to the Castilians. This is the official motive behind Afonso's next action: he sent three men to find Inês and murder her in 1355. Pedro's rage at the murder of his love is what allegedly sparked his revolt against his father. This revolt began in 1355 and lasted into 1356, when Afonso finally defeated his son. One year later Afonso died, and Peter succeeded to the throne. "La Reine morte", a French drama was inspired by the conflict between King Afonso and Pedro and Ines.

It is possible that Peter I of Portugal has been confused with Peter of Castile: both have the same name, both lived at the same time, the two were closely related, and both are credited with committing violent acts towards their subjects. Despite his gruesome legacy, Peter I of Portugal did have a peaceful reign and managed to install a system of justice which was relatively fair for the times. He attempted this with his Beneplácito Régio in 1361, which forbade any Papal Bulls to be published without his prior consent. This was a result of the number of fake papal documents that had been entering the country. He also began the "nationalization" of the military orders by placing his youngest son João (an illegitimate child born after Inês' death) as the Master of the Order of Avis. He claimed that he and Inês had been married and thus that their four children were legitimate, but nothing ever came of this. Peter and Inês' children went to live in Castile.

Legend holds that Peter later had Inês' body exhumed and placed upon a throne, dressed in rich robes and jewels, requiring all of his vassals to kiss the hand of the deceased "queen". This has never been proven, but what is known is that Peter did have Inês' body removed from her resting place in Coimbra and taken to Alcobaça where her body was laid to rest in the monastery. Peter had two tombs constructed in the monastery, one for each of them. These still exist today; they contain images of Peter and Inês facing each other, with the words "Até o fim do mundo..." or "Until the end of the world..." inscribed on the marble.

Peter was also the father of Ferdinand I of Portugal and John I of Portugal. John was the Master of the military order of Avis, and he would become the founder of the Avis dynasty in 1385, after defeating an attempt by Beatrice of Portugal and John I of Castile to ascend the Portuguese throne.