His early studies were at the convent of San Francisco, after which he continued those studies at the legal practice of Manuel José de Labardén. In 1789 he was appointed notary general for the bishopric, and held that post until the events of May, 1810. He was unaware of the impending revolution and was caught by surprise when the town hall was occupied on May 10, 1810; he did not agree that it had been legitimately occupied. His donations to the Sociedad Patriótica associated him with the Saavedrist faction, and the leaders of the riots of April 5, 1811 sent him to Mendoza. A month later he was appointed solicitor-procurator for the city of Buenos Aires.
The second triumvirate named him with Nicolás Rodríguez Peña and Juan Larrea to draft a constitution for consideration by the Assembly for 1813, and he became a part of that triumvirate when the Assembly granted it executive power. Shortly afterwards, on January 22, 1814 they decided to concentrate power in him as Supreme Director of the United Provinces, and he took the post soon after. During his short rule Saavedra and Campana were exiled, there was dissidence with Artigas, Montividep fell, and Ferdinand VII of Spain regained his throne. With the fall of the Alvear faction in August 1815, Posadas was jailed, and he occuped 22 different jails in the following six years. He began writing his memoirs in 1829.