He also had an outstanding participation as a landowner, dedicated to cimarron cattle, being one of the most powerful accioneros (hunting) of South America. His parents belonged to noble families of the Iberian peninsula, descendants of the Royal House of Portugal and Castile.
Manuel was born in Vila do Porto, São Miguel Island, son of Amador Vaz de Alpoim and Margarida Cabral de Melo, belonging to the Portuguese nobility. In 1598 he arrived at the Río de la Plata in the company of their parents. His family had lived for five years in the Portuguese colony of Brazil, in which they had taken refuge after the earthquake in the Azores islands of 1591.
He began his studies in Buenos Aires, to continue his education at Lisbon (Portugal). In 1613, he returned to Buenos Aires, and began to work in his father's business. By the year of 1617 his father dies in Buenos Aires, and his mother towards the year of 1630.
During the decades of 1620 and 1630s, Manuel Cabral de Melo e Alpoim occupied several government posts, including the positions of alcalde, regidor and Maestre de Campo. 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. Later he held the position of Regidor of the Cabildo de Buenos Aires, having an active participation in matters related to the Río de la Plata.
Resident in Corrientes he was appointed as Lieutenant Governor of Province in 1629. 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. Commanding an army of 200 soldiers of Guarani origin, departed from the city of Itatí to Rio Grande do Sul, in support of the Jesuits. The rapid intervention of Alpoim, saved the lives of hundreds of people. Also gave approximately 40.000 cattle for creating stays in the area.