He reigned briefly from 26 June 1409 to his death in 1410, in opposition to the Roman pope Gregory XII and the Avignon antipope Benedict XIII.

He became an adviser to, Giangaleazzo Visconti, the Duke of Milan, and for almost ten years, Peter was prime minister to Galeatus and his son.

[2] He returned to Lombardy, where, thanks to the favour of Giangaleazzo Visconti, Peter became bishop, first of Piacenza (1386), then of Vicenza (1387), then of Novara (1389), and finally Archbishop of Milan (1402).

[3] During his ten-month reign, Alexander's aim was to extend his obedience with the assistance of France, and, notably, of Duke Louis II of Anjou, upon whom he conferred the investiture of the Kingdom of Sicily, having removed it from Ladislaus of Naples.

He proclaimed and promised rather than effected a certain number of reforms: the abandonment of the rights of "spoils" and "procurations", and the re-establishment of the system of canonical election in the cathedral churches and principal monasteries.