Justino Diaz first studied at the University of Puerto Rico. He made his operatic debut in 1957 as Ben in Menotti's The Telephone, and later studied with Frederick Jager and at the New England Conservatory of Music in Boston. After winning the Metropolitan Opera Auditions of the Air he made his debut at the Met as Monterone in Rigoletto in 1963, beginning a long-term association with that house. He has sung thirty roles and given nearly 300 performances there. In 1966, he created the role of Antony in Barber's Antony and Cleopatra on the occasion of the opening of the new opera house. In the same year, he made his Salzburg debut as Escamillo in Carmen. In 1971, he created another new role at the inauguration of another opera house, this time as Francesco in Ginastera's Beatrix Cenci at the Kennedy Center Opera House in Washington, D.C. His Covent Garden debut was also as Escamillo, in 1976, and his La Scala debut was as Asdrubale in Rossini's La pietra del paragone in 1982. He also appeared in the Zeffirelli film of Verdi's Otello in 1986.