Prieto was born in Mexico City, the son of José María Prieto Gamboa and Josefa Pradillo y Estañol. His childhood was spent near Molino del Rey (King's Mill), next to the historic Chapultepec Castle, since his father administered the mill and the associated bakery. When Prieto was 13 his father died and his mother had a nervous breakdown. Andrés Quintana Roo and Fernando Calderón took him under his protection, and he was thus able to continue his studies. After working in a clothing store and in the customs, he entered the Colegio de San Juan de Letrán.