He was accused of cowardice by the people of Buenos Aires after escaping the city during the British invasions of the Río de la Plata in 1806.

At fourteen years of age, he became a cadet in the Regimiento de las Reales Guardias Españolas.

Created the Civil Law career at the University of San Carlos, improved the administration of the neighborhoods, started the first street lighting system and founded a women's hospital.

In that capacity he labored to improve it to be able to resist an invasion from Brazil or England, specifically fortifying Montevideo and Colonia del Sacramento.

He requested help from the Spanish Courts, but the Prime Minister Manuel Godoy answered that he should defend as best as he could, with no help forthcoming.

The Viceroy understood that arming the creole civilian population, many of them influenced by revolutionary ideas, fomented by the American and French revolutions, was a dangerous strategy for the interests of the Crown.

As part of his defensive measures, he made a highly experienced officer of the Spanish Navy, Santiago de Liniers, the commander of the port of Ensenada de Barragán, about 70 km (43 mi) south of Buenos Aires, with orders to protect the coast.

On 24 June 1806, while attending a theater play with his family, the viceroy received news that British ships have been sighted along the coast.

Sobremonte left the play early going to Buenos Aires Fort, where he wrote an order to organize the defense.

The next morning, the enemy's ships were sighted again on the Buenos Aires coast and cannon was shot from the fort with no effect.

Since the time of Vértiz there was a regulation that stated that if Buenos Aires was attacked by a foreign invader and the capital could not be held, there must be a move to the interior of the country and organize the defense in Córdoba, in order to defend the rest of the Viceroyalty, and have a fighting chance to reconquer the capital with some chances of success.

The invaders captured the treasure, sending it to London, where it was paraded in triumph on its way to the bank vaults (not knowing that a month earlier, the porteños had reconquered the city).

Immediately after the reconquest, Álzaga called for an open cabildo of the people of Buenos Aires to refuse the viceroy command of the city.

When Montevideo fell to the British, the open cabildo in Buenos Aires deposed him as viceroy, replacing him with Liniers.