Alfonso de Cartagena was the second son of Rabbi Paul of Burgos, who converted from Judaism to Christianity in 1390 or 1391. At the same time, Alfonso and his four brothers, one sister and two uncles were baptized. His mother, however, was not. Cartagena studied law in Salamanca, and "was a great lawyer in canon and civil law", according to Claros varones de Castilla (1486). He served as dean of Santiago de Compostela and Segovia, later becoming apostolic nuncio and canon of Burgos (1421).