Peter was born at Atlit, Kingdom of Jerusalem, while his father led the Seventh Crusade. Back in France, he lived in Paris until 1269 when his father gave him in appanage the County of Alençon. He accompanied his father to Tunis during Eighth Crusade (1270), but this expedition was a fiasco, because of the dysentery epidemic that decimated the army of crusaders. His father and his brother Jean Tristan succumbed to the disease.