Sánchez was chosen to succeed Antipope Benedict XIII (Benedict (XIII)). Refusing to recognize the Roman pope Martin V during the Western Schism, Benedict created his own cardinals, who, through the influence of King Alfonso V of Aragon, chose Sánchez at the castle of Peñíscola, in Valencia, as Clement VIII on June 20, 1423. Benedict's cardinal Jean Carrier vehemently opposed the choice, declaring Sánchez vile and his election invalid. Clement postponed his coronation until May 19, 1426.

Alfonso despised Martin and used Clement against him. When Alfonso was later reconciled with Martin, however, the defenseless Clement abdicated on July 26, 1429, revoked all his decrees, and set forth his case: he established his acceptance of his election as the only hope to secure church peace and stressed that his abdication was his choice only. He asked the church to recognize Benedict's validity, or else his own election and abdication would be without purpose. After retiring to San Mateo, he had his cardinals acknowledge Martin as pope, thus ending the Western Schism. Absolved from censure and reconciled with the church, Sánchez was elected bishop of Majorca.