Prieto was one of five sons of a Creole officer in Concepción. After finishing school, he joined the cavalry garrison in his home town. In 1810, he joined the Chilean fight for independence against his father's will. He met Manuela Warnes García de Zúñiga in Buenos Aires and married her in 1812. During the Chilean War of Independence, he served as a captain. In the dispute between Bernardo O'Higgins and José Miguel Carrera, he took the side of O'Higgins, who then made him Quartermaster general of the southern army.