Dorrego was born in Buenos Aires on 11 June 1787 to José Antonio do Rego, a Portuguese merchant, and to María de la Ascensión Salas.

[1] He moved to the United Provinces of the Río de la Plata (modern Argentina), and joined the Army of the North, under the command of Manuel Belgrano.

As a result, he did not take part in the battles of Vilcapugio and Ayohuma, two defeats of the Army of the North, and Belgrano regretted later the absence of Dorrego from them.

[1] Dorrego opposed the Luso-Brazilian invasion of the Banda Oriental, encouraged by Juan Martín de Pueyrredón to counter the influence of José Gervasio Artigas.

[4] Dorrego opposed the government of the unitarian Bernardino Rivadavia, who was appointed as the first president of Argentina, and voiced his criticism in the newspaper El Tribuno.

[5] The Argentine troops were discontented with Dorrego because he accepted the conditions imposed by the British diplomacy despite their military victories in the conflict.