Peter first appeared in contemporary records in May 1275, when he was elected Bishop of Veszprém. Prior to that, his family had no interests in Veszprém County, but the Ugod branch of the Csák kindred possessed landholdings and castles in the region. Peter's election occurred under political pressure, when the Kőszegis regained their influence in the royal council by mid-1275. Under such circumstances, the town of Veszprém and the surrounding lands became a buffer zone between the two rivaling families, the Kőszegis and the Csáks. In early 1276, Peter Csák and his allies launched a massive military campaign against the Kőszegi and Gutkeled dominions. Their troops plundered and devastated the territory of the Diocese of Veszprém. During the attack, 68 canons, priests and clergymen were killed, several others were tortured and robbed, while all the treasures of the Veszprém cathedral chapter, including the library and archives of its school were looted and burnt. The canonical university was never rebuilt after Peter Csák's campaign. According to contemporary records, this was "the worst barbaric and most destructive attack" in Hungary since the Mongol invasion. Despite that, Peter Kőszegi retained his position. His election was confirmed by the pope in mid-1276. He was also styled traditionally as chancellor of the Queen Elizabeth of Sicily in 1277, and later from 1284 to 1285.