Returning to America, he made his debut at the Metropolitan Opera in excerpts from La Traviata and Pagliacci during a concert in New York in November 1938. His formal operatic debut took place there in January 1939, when he sang Paolo in Simon Boccanegra. He quickly advanced in public favour, eventually assuming a leading place among the noted baritones of his time. He also sang in San Francisco, Chicago, Canada, and South America. He appeared at La Scala in Milan in 1953. In 1958 he made a highly successful tour of the Soviet Union. His last complete performance at the Metropolitan Opera was as Simon Boccanegra on March 1, 1960, 3 days before his tragic death.
He was particularly acclaimed as one of the foremost interpreters of the great Verdi baritone roles; he also sang the parts of Tonio in Pagliacci, Escamillo in Carmen, and Scarpia in Tosca. He collapsed while singing the aria "Urna fatale dal mio destino;" as Don Carlo during a performance of La forza del destino, underlining the tragic irony of the words, and died of a cerebral hemorrhage backstage. He was reputed to be a person of an intractable character, who always tried to impose his will on stage designers, managers, and even conductors, in matters of production, direction, and tempi. He caused pain, a colleague said, but he had a great voice.
Find out more at https://en.wikipedia.org/wiki/Leonard_Warren