 De Mafra was born in the town of Mafra, north of Lisbon, Portugal, or, according to other sources, in the town of Jérez de la Frontera, in Cadiz, Andalusia.In 1519, he became a crew member of the Magalhães expedition.De Mafra started as a seaman in the galleon "Trinidad", the armada's flagship, and was on board when the Portuguese captured the Spanish vessel at Benaconora, today Jailolo, in the Moluccas.