Robert was born in the Château d'Annecy in 1342, the son of Amadeus III, Count of Geneva, and Mahaut de Boulogne,[2][3] who were important within the House of Savoy.

[6] In 1377, while serving as papal legate in upper Italy (1376–1378), in order to put down a rebellion in the Papal States,[7] known as the War of the Eight Saints, he personally commanded troops lent to the papacy by the condottiere John Hawkwood to reduce the small city of Cesena in the territory of Forlì, which resisted being added to the Patrimony of Peter for the second time in a generation; there he authorized the massacre of 3,000–8,000 civilians, an atrocity even by the rules of war at the time, which earned him the nickname butcher of Cesena.

[10] Following a victory at Marino by Urban VI's troops,[11] Clement, feeling vulnerable, fled Anagni to Sperlonga, then Gaeta, finally landing at Naples.

[12] Received with great respect by Queen Joanna I of Naples, Clement found himself assailed by the local populace which chanted, "Viva Papa Urbano" and "Muoia l'Anticristo".

[18] Coupled with the expectation of succeeding to Queen Joanna, Clement incited Louis I, Duke of Anjou, the eldest of the brothers of Charles V, to take arms in his favour.

Still, these enterprises on several occasions planted Angevin domination in the south of the Italian peninsula, and their most decisive result was the assuring of Provence to the dukes of Anjou and afterwards to the kings of France.

[15] There came a time, however, when Clement and more particularly his following had to acknowledge the vanity of these elusive dreams; and at the end of his life he realized the impossibility of overcoming by brute force an opposition which was founded on the convictions of the greater part of Catholic Europe.