After completing his studies, he occupied several military and government posts, including the positions of alcalde, regidor and Maestre de Campo.

[7] His first public office was in 1621, when he was appointed as Alcalde de la Hermandad, serving in the suburban and provincial area of Buenos Aires.

[10] He led the crackdown against the rebel Indians, who in 1628, had murdered the Spanish Jesuits among them Roque González de Santa Cruz, cowardly attacked by the cacique Ñezú in reduction of San Nicolás, located across the Uruguay River.

[12] The heart of Roque González de Santa Cruz, burned and pierced with an arrow, was found by Alpoim inside a bag of relics.

In 1620 he had obtained permission for the exploitation of "ganado cimarron" (wild cattle and horses), of great abundance in the areas of the Province of Buenos Aires.

[20] Manuel Cabral de Melo y Alpoim had an active participation as a member of the provincial militias of the Spanish army in Buenos Aires.

[24] His mother Margarida Cabral de Melo, was directly related to the Portuguese Royal houses by paternal and maternal line.

Among his illustrious ancestors include the kings Afonso III of Portugal[25] and John of England, through of Álvaro Martins Homem, a great-great-grandfather of Manuel Cabral de Melo y Alpoim.

[27] Manuel Cabral de Melo e Alpoim claimed that his direct paternal ancestors were medieval knights of France,[28] probably of Norman origin.