He was born in Crete of unknown parents and entered the Franciscan order. His abilities were such that he was sent to study at the universities of Oxford and Paris (Sorbonne), while in Paris the Great Schism (1378-1417) occurred, Philarges supported Urban VI. He settled in Lombardy, where, thanks to the favour of Giangaleazzo Visconti the Duke of Milan, he became bishop, first of Piacenza (1386), then of Vincenza (1387), then of Novara (1389), and finally archbishop of Milan (1402).
On being created cardinal by Innocent VII in 1405 he devoted all his energies to the re-union of the church, in spite of the two rival popes. He was one of the promoters of the Council of Pisa and his politicing incurred the displeasure of Gregory XII, who ordered Philarges derived of both his archbishopric and his cardinalitial dignity.
At the Council of Pisa (from March 25, 1409) the assembled cardinals chose Philarges as the new prelate for a chair they presumed was vacant. He was crowned on June 26, 1409 making him in reality the third rival pontiff.
During his ten month reign Alexander V's aim was to extend his obedience with the assistance of France, and, notably, of the Duke Louis II of Anjou, upon whom he conferred the investiture of the Kingdom of Sicily, having removed it from Ladislas of Naples. He proclaimed and promised rather than effected a certain number of reforms: the abandonment of the rights of "spoils" and "procurations," the re-establishment of the system of canonical election in the cathedral churches and principal monasterie. He also gave out papal favours with a lavish hand, the mendicant orders benefitted especially.
Death came upon him suddenly while he was with Cardinal Baldassare Cossa at Bologna, in the night of the 3rd-4th May 1410. His remains were placed in the church of St. Francis at Bologna. A rumour went about that he had been poisoned by Cossa, who did succeed him as John XXIII. Whether Alexander V was a pope or an antipope is still a matter of debate.