Like many of the great Otellos (including Ramon Vinay and Plácido Domingo), Zanelli started his career as a baritone and developed into a tenor during the early years of his career. Throughout his career, he retained a richness of timbre that served him well in dramatic roles, and the secure bottom notes that helped make him "the new Tamagno," particularly as Otello. His acting was as acclaimed as his singing, and he had a particularly commanding stage presence, well-suited for heroic roles. He studied privately with Angelo Querenz and made his opera debut in Santiago as Valentin in Gounod's Faust in 1915. He followed this with his Met debut as Amonasro in Verdi's Aida in 1919, and in 1923, following a suggestion from Arturo Toscanini, went to Italy where he studied to sing as a tenor. His debut as a tenor was in Naples as Raoul in Meyerbeer's Les Hugenots, and he continued to sing lyric roles until realizing that his voice was a dramatic tenor. He studied Otello under Leopoldo Nugnone, and sang the role for the first time in Turin in 1926, repeating it for his 1928 London debut. In 1930, he appeared in the world premiere of Pizzetti's Lo Straniero, and made his La Scala debut in 1932 as Otello. He was also adding Wagner roles to his repertoire, centered around the more lyrical Lohengrin, Siegmund, and Tristan. During this time, he was already fighting cancer, sang his last Otello in 1933, and died in 1935. His brother was the noted baritone Carlo Morelli (who mentored Domingo in his transition to tenor).