While he studied music as a youngster, Kraus had no intention of becoming a professional singer, until friends and family began encouraging him to do so. In taking up vocal studies, Kraus avoided heavier repertory and focused on the bel canto tenore di grazia parts that he knew were right for his voice. At the age of 28, he won first prize at the Geneva Competition, and a representative of the Cairo Opera, who was present at the auditions, offered him the role of the Duke in Rigoletto; he made his professional opera debut at the Cairo Opera in that role in 1956. His great success there was followed by equally gratifying appearances in Venice, Turin, and Barcelona, and in 1958, he appeared in La Traviata with Maria Callas in Lisbon -- the Lisbon Traviata. In 1959, he sang Arturo in I Puritani for the first time, made his La Scala and Covent Garden debuts, followed by his Metropolitan Opera debut in 1965 and Salzburg debut in 1968. In Rome, he sang his first Werther, a role that, like Arturo, was to become one of his signatures.
Aside from his fine sense of the musical nuance and phrasing, his portrayal of the mentally unstable, morbid, masochistic, and manipulative character of Werther has been acclaimed as one of the most effective and insightful readings ever. During the 1980s, he began to limit the number of his performances even further (at the peak of his career, he never sang more than 50 in a year), and started to turn his attention to teaching, although even in the 1990s he still had an active performing schedule. Kraus died on September 10, 1999, after an extended illness.