His birthplace Santo Ángel was, at that time, part of Misiones Province, but currently belongs to the Brazilian state of Rio Grande do Sul.

He returned to Buenos Aires on board the British merchantman George Canning, in which were also travelling San Martín, Juan Matías Zapiola, Francisco Chilavert and other soldiers.

Alvear was a leader of the constituent Assembly of the year 1813 and, goaded by political ambition, succeeded in establishing a Unitarian (centralizing) form of government, having his uncle Gervasio Antonio de Posadas named Supreme Director (chief executive).

A few months later, he replaced General José Rondeau as commander in chief of the army besieging Montevideo, the last bastion of Spanish power in the River Plate, which was defended by 5,000 troops.

In late June 1814, as news that Ferdinand VII had recovered the crown of Spain, Alvear managed to force the surrender of the Spanish troops in Montevideo.

At the end of 1814 Alvear was named commander of the Army of the North, but he lacked of support from Posadas, and his unpopularity among the troops, and other disagreements—including a project for a constitutional monarchy that he sent to Europe to be negotiated by Manuel Belgrano, that was fiercely opposed by the League of Free Peoples—made him return to Buenos Aires.

Having neither the support of the troops nor sufficient influence on the people of the hinterland provinces, Director Alvear then attempted to come to an alliance with Artigas, to whom he offered the independence of the Banda Oriental (current Uruguay).

In May of that year, he moved to Montevideo where he joined his friend, the Chilean Jose Miguel Carrera, also exiled due to political differences with San Martin and Bernardo O'Higgins.

In 1825, together with José Miguel Díaz Vélez, Carlos María de Alvear was sent by the Buenos Aires government to Bolivia to meet with Simón Bolívar.

[4] The real objective of the so-called Alvear-Díaz Velez Mission was to seek Bolívar's support in the looming war with the Empire of Brazil, over the Banda Oriental.

It was proposed that Bolívar should lead a Hispanic American alliance that can exert pressure on Dom Pedro I to withdraw his army stationed in the Eastern Province.

[4] The mission, which was created by an Act passed on May 9, 1825, gave Alvear and Díaz Vélez authority to negotiate and settle issues involving the liberation of the four provinces of Upper Peru.

During his residence in the United States, Alvear had the opportunity to meet and interact with important political figures such as Joel Roberts Poinsett, Daniel Webster, John Calhoun and James Buchanan, among others.

[citation needed] Although he had been a lifelong admirer of the United States, after the annexation of Texas (1845) and the subsequent war with Mexico (1846–1848), Alvear became wary of American intentions towards Spanish America.

According to his American biographer Thomas Davis, his diplomatic correspondence shaped Argentina's traditional distrust to U.S. policies, which Alvear felt included the desire to conquer, or at least dominate, all of Latin America.

Revisionist authors, supporters of anti-imperialism, condemn Alvear for the attempt to turn the United Provinces into a British protectorate and relate him with the party of Bernardino Rivadavia, despite them being enemies.