He was unaware of the impending revolution and was caught by surprise when the Buenos Aires Cabildo (town hall) was occupied on 25 May 1810; he did not agree that it had been legitimately done.

His donations to the Sociedad Patriótica made him an associate of the Saavedrist faction, so the leaders of the riots of 5 April 1811 exiled him to Mendoza.

Then on 22 January 1814 the same Assembly decided to concentrate the Executive Power in him as a Supreme Director for the United Provinces, and so he took that office for a one-year period.

During his rule, Saavedra and Campana were exiled, Montevideo fell to the United Provinces but serious problems arose with José Gervasio Artigas and the Liga Federal on the Banda Oriental.

Posadas was succeeded in office by his nephew, Carlos María de Alvear, who was removed soon afterwards by a military coup d'état.