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. He enrolled in the "Real Colegio de San Carlos" in 1803, and moved to the "Real Universidad de San Felipe" in the Captaincy General of Chile to continue his studies. He supported the early steps of the Chilean War of Independence in 1810, which led to the removal of the Spanish colonial authorities and the establishment of the first Chilean Government Junta.