Peter was the fourth of five sons of Count Amadeus III of Geneva

Amadeus III of Geneva

Amadeus III was the Count of Geneva from 1320 until his death. He ruled the Genevois, but not the city of Geneva proper, and it was during his time that the term "Genevois" came to be used as it is today. He was the eldest son and successor of William III and Agnes, daughter of Amadeus V of Savoy...

 and succeeded his brother John I as Count of Geneva

Count of Geneva

The County of Geneva, largely corresponding to the later Genevois province, originated in the tenth century, in the Burgundian Kingdom of Arles which fell to the Holy Roman Empire in 1032.-History:...

 in 1370. When he died without a son to succeed him in 1392, the county passed to the fifth of the brothers, then Antipope Clement VII

Antipope Clement VII

Robert of Geneva was elected to the papacy as Pope Clement VII by the French cardinals who opposed Urban VI, and was the first Avignon antipope of the Western Schism.-Biography:...

.

Peter led a contingent of Genevans in an invasion of the Kingdom of Naples

Kingdom of Naples

The Kingdom of Naples, comprising the southern part of the Italian peninsula, was the remainder of the old Kingdom of Sicily after secession of the island of Sicily as a result of the Sicilian Vespers rebellion of 1282. Known to contemporaries as the Kingdom of Sicily, it is dubbed Kingdom of...

 in 1382. The invasion was led by his lord, Amadeus VI, Count of Savoy

Amadeus VI, Count of Savoy

Amadeus VI , nicknamed the Green Count was Count of Savoy from 1343 to 1383. He was the eldest son of Aimone, Count of Savoy and Yolande of Montferrat....

, and by Louis I, Duke of Anjou, the adopted son and heir of the imprisoned Queen Joan I of Naples

Joan I of Naples

Joan I , born Joanna of Anjou, was Queen of Naples from 1343 until her death. She was also Countess of Provence and Forcalquier, Queen consort of Majorca and titular Queen of Jerusalem and Sicily 1343–82, and Princess of Achaea 1373/5–81....

, all supporters of his brother's claim to the papal throne. He was present at the deathbed of Amadeus at Castropignano

Castropignano

Castropignano is a comune in the Province of Campobasso in the Italian region Molise, located about 10 km northwest of Campobasso....

on 1 March 1383.

Around 1377 Peter was employing a Jewish physician, Isaac de Portis.