In September 1953, he made his American debut at the San Francisco Opera in the title role of Werther, opposite Giulietta Simionato as Charlotte, and later that same year, in December, he debuted at the Metropolitan Opera in New York, singing Don Ottavio in Don Giovanni.

Other roles at the Met included Tamino, Almaviva, Nemorino, Ernesto, Alfredo, and des Grieux.

His career at the Met ended abruptly in November 1960 when manager Rudolf Bing replaced Valletti, for no apparent reason, after dress rehearsal in a new production of L'elisir d'amore.

In May, 2000, Valletti died of a heart attack in Genoa, Italy, while undergoing treatment for liver cancer.

Valletti can also be heard in a best-selling RCA Victor recording of Madama Butterfly which paired him with Anna Moffo.