He held the lordships (señoríos) of Cameros, Almazán, Berlanga, Monteagudo and Cifuentes and was the majordomo of his brother, King Ferdinand IV of Castile.

During the minority of his nephew Alfonso XI, he shared the regency of Castile with his mother and uncle, John el de Tarifa, between 1313 and his death.

Throughout the disputes over the regency in the years 1312–15 that often devolved into open conflict, Peter supported his mother and was often at odds with his uncle.

In 1317, he again invaded Granada, relieving the siege of Gibraltar and capturing the castle of Bélmez de la Moraleda.

In 1319, having received papal approval for a crusade against Granada, Peter and John jointly led an expedition to besiege the fortress of Tíscar.