Before turning 30, Viera-Gallo was a Member of Congress during the left-wing elected government of president Salvador Allende, representing the Socialist Party of Chile.

Like other left-wing ousted politicians, he sought refuge in a foreign diplomatic mission, specifically the Apostolic Nunciature, where he lived for over six months.

With the help of then Apostolic Nuncio Angelo Sodano (who would later rise to the rank of cardinal in the Catholic hierarchy), Viera-Gallo was able to obtain safe passage out of Chile, and moved to exile in Rome, Italy.

Together with other politicians including Bernardo Leighton, and with the support of leading Italian socialists including Bettino Craxi and Rino Formica, he led the Socialist Party of Chile from exile in Italy, while the Communist Party of Chile was led by Colodomiro Almeida and others from Moscow in the then Soviet Union.

As part of his advocacy efforts to fight the regime of Augusto Pinochet, Viera-Gallo founded the magazine ChileAmérica, which would later become incorporated into the Centre for Social Studies (CESOC).