Born in Piraeus, Zaccaria studied at the Athens Conservatory where he enjoyed his debut in 1949, aged 26. He sang at La Scala in 1953 and his position as a mainstay of the bass operatic repertoire was assured thereafter. He was La Scala's principal bass for almost 15 years. He sang with some of the most famous singers of his generation, such as Maria Callas,Leontyne Price, Franco Corelli, and Marilyn Horne, who was Zaccaria's companion in later life. Despite intimidating competition, he developed an impressive international career and recorded more than 30 operas for major recording companies. With Callas he recorded nine complete operas:
According to John Ardoin in his book The Callas Legacy, Zaccaria also recorded under the pseudonym Giulio Mauri in the complete recordings of Il trovatore and Turandot in which he appeared with the soprano.