Navarro was probably born at Garde in the Navarrese valley of Roncal. Little is known of his early life. He began his military career in the service of Cardinal Juan de Aragon prior to 1485. He fought against the Barbary pirates in Italy as a Condottiere. Enlisted by Gonzalo Fernández de Córdoba in 1499, he took part in the capture and siege of Cephalonia in 1500. He invented the landmine and his skilful employment of mines allowed for the breaching of the walls of the Turkish fortress. He continued in the service of Gonzalo Fernández de Córdoba and went on to Naples, and defended Canosa in 1502 and Taranto in 1503 against the French. He supervised the construction of the field fortifications at the Battle of Cerignola, that enabled Gonzalo Fernández de Córdoba to win his battle with Louis d'Armagnac, Duke of Nemours on 28 April. He played a major role in the Spanish victory at the Garigliano River on 29 December 1503 and was created count of Oliveto for his services. After returning to Spain in 1507 he took part in the North Africa expeditions of Cardinal Cisneros. Navarro assisted in the capture of the Peñón de Vélez de la Gomera in 1508 by employing a floating battery of his own design during the battle. He went on to fight at the capture of Mazalquivir (Mers-el-Kébir) and Oran in 1509. Navarro personally led the Spanish forces during the conquest of Bougie (Béjaïa), Algiers, Tunis, Tlemcen, and Tripoli in 1510. He enlisted in the service of Ramon de Cardona viceroy of Naples and returned to Italy upon hearing of a new war against France in 1511. He constructed a number of light carts mounted with light artillery pieces designed to break up enemy formations in 1512.