He had a long career in legal matters in the Río de la Plata. In 1719, he served as a defensor de pobres (defender of minors), and like lawyer or adviser of Buenos Aires. He also was appointed to the position of conjuez de menores (judge of minors) in 1745. In 1731, Esparza was designated as Fiel Ejecutor, a charge that consisted in the inspection and control of commercial activity in the Río de la Plata. In 1746, he served temporarily as alcalde ordinario of the city of Buenos Aires in substitution of Gaspar de Bustamante, alcalde of second vote. He returned to occupy the post of mayor of second vote in 1760, this time in substitution of Don Joseph de Iturriaga. In 1744, he was appointed by the governor Domingo Ortiz de Rosas, to take charge of the completion of the padron (census) of the city of Buenos Aires. Miguel Gerónimo de Esparza was also in charge of providing services in ceremonies and public events in Buenos Aires. For several periods it was designated as Alférez Real, in charge to attend the patronal party in honor of San Martín de Tours, patron of the City and the archdiocese of Buenos Aires. In 1725, he took part in the senses tributes that were made in the city to the Monarch Louis I of Spain, died on August 18, 1724. In 1733 he participated in the festivities of the city on the occasion of the Conquest of Oran by the Spanish Empire. Like many politicians of the time Esparza, was also dedicated to the military career. He took part in various military campaigns against the incursions of the Pampas, including his actions against the tribes of the Calelián chief in the northern territories of the Province of Buenos Aires. He also served as contador del Ramo de Guerra (Council of War) and as a commander in the Fuerte de Buenos Aires, the main fortification of the city. He was also dedicated to commerce, he belonged to the select group of neighbors of Basque and Creole origin, who had ties to Cadiz. Among these neighbors was Juan de la Palma y Lobatón, Domingo de Basavilbaso, his colleagues in the Ayuntamiento, and Alonso García de Zúñiga, belonging to one of the most powerful families of Río de la Plata.