After the death of his father in 1391, his mother acted as a regent, because of his youth.
In 1418 his distant cousin Louis of Savoy-Achaea, his brother-in-law, the last male of the elder branch of House of Savoy, died, leaving Amadeus as his heir-general, thus finally uniting the male-lines of the House of Savoy.
He increased his dominions, encouraged several attempts to negotiate an end to the Hundred Years' War, and, after the death of his wife, retired from his position as Duke to become a hermit. Elected by the Council of Basel as an alternative Pope in opposition to Pope Eugene IV, Amadeus served as Felix V, regarded as an antipope, before stepping down to accept a Cardinal's hat instead.
Amadeus' image in history is marred by the account of him as a pontiff concerned with money, to avoid disadvantaging his heirs, found in the Commentaries of Pius II. Nor is there evidence that he intrigued to obtain the papal office, sending the bishops of Savoy to Basel for this purpose.
(Note on numbering: When numbering of the Popes began to be used, Antipope Felix II was counted as one of the Popes of that name. The second true Pope Felix is thus known by the number III, and the true third Pope Felix was given the number IV. It also affected the name taken by Amadeus, who would have been the fourth Pope Felix.)