Zenatello showed musical promise from a young age. His singing teacher in Verona originally trained him as a baritone but he never felt comfortable in this range and later switched to the higher tenor register. Nonetheless, it was as a baritone that he made his professional debut at Belluno in 1898. His tenor debut—as Canio—did not come until the following year, at Naples.
Zenatello's operatic career gathered momentum during the early years of the 20th century, and on 17 February 1904 he created the role of Pinkerton in the world première of Giacomo Puccini's Madama Butterfly at La Scala, Milan. La Scala was Italy's leading opera house, and he remained a member of its company of singers until 1907.
He also sang at the Royal Opera House, Covent Garden, London, in 1905/06 and 1908/09; at the Manhattan Opera House in 1907 to 1910; and with the New York Metropolitan Opera company, on tour, replacing Enrico Caruso, in 1909. He was a member of the Boston Opera Company from 1909 to 1914 and sang often, too, in South America and Mexico and in various cities of continental Europe.
Zenatello's voice had matured into that of a clarion-voiced dramatic tenor during his La Scala years, and he succeeded Francesco Tamagno (1850–1905) as the world's greatest exponent of Giuseppe Verdi's Otello. He performed this extremely taxing role more than 300 times, beginning in 1908, and recorded highly acclaimed extracts from the work on 78-rpm discs.
Zenatello returned to Covent Garden in 1926, singing Otello in a series of performances which were partly recorded live by HMV. After a final stage appearance (in New York City) in 1933, he retired from opera and taught singing and dabbled in the management of promising young singers. The coloratura soprano Lily Pons was a notable 'discovery' of Zenatello's in the twilight of his on-stage career. He took Pons under his wing and arranged for her an audition before the Met's general manager, which resulted in the young French-born singer making a successful New York debut in 1931. Later, however, they fell out.
Another great pupil of his was the Veronese tenor Nino Martini who had his debut at the Metropolitan with Rigoletto in 1933, which enabled him to start his extraordinary career at the Metropolitan (1933-1946), in the greatest American theatres and at Hollywood as well.
As to his private life, Zenatello lived with the Spanish mezzo-soprano Maria Gay from 1906 until her death in 1943. They were often described as husband and wife although they may never have actually married. He and Gay performed opposite each other on many occasions and settled down together in Manhattan in 1936.
Back in 1913, Zenatello had been instrumental in having the Verona Arena, built originally by the ancient Romans, restored and turned into a world-famous open-air venue for operatic performances. That same year the arena was used to mount a grand production of Aida, marking the centenary of Verdi's birth. In 1947, Zenatello arranged for a promising young soprano named Maria Callas to appear at the arena's Summer Festival in Amilcare Ponchielli's opera La Gioconda. This appearance would give Callas invaluable exposure in Italy and help set her on the path to future stardom.
Zenatello died of natural causes in New York City in 1949, aged 73.