Martinelli remains a legend of stamina and longevity in the opera world, particularly for a heroic tenor, a type of voice not always associated with longevity. He made his opera debut in 1908 and sang his last performance in 1967. His voice was not, by contemporary accounts, as huge as that of most heroic tenors, but he had such strong focus and projection that he more than compensated for this perceived shortfall. Particularly at the Met, Martinelli was considered Caruso's successor in the more dramatic roles, as Gigli was in the more lyrical ones. Martinelli had a strong sense of legato phrasing, powerful breath control, and a distinctive timbre, although some listeners found it overly metallic. Martinelli made more use of rubato than what would be permitted in post-1970s practice, but unlike most heroic tenors of any era, he generally sang, rather than slurred, grace notes.
The eldest of 14 children (which may well have been a driving force behind developing such a strong voice!), Martinelli sang in the church choir and played the clarinet. When Martinelli was 17 and serving in the military, his powerful voice drew the attention of an officer who arranged for him to study with Maestro Mandolini, a noted teacher in Milan. Martinelli's opera debut in 1908 was as the messenger in Aida. In 1910, Martinelli sang the title role of Ernani at La Scala, and was promptly invited to audition for the Italian premiere of La Fanciulla del West (which had premiered at the Met with Caruso.) After some hesitation over his lack of experience, Puccini and Toscanini chose him, and later, according to one story, he was Puccini's choice for the world premiere of Turandot, but the Met management would not release Martinelli from his contract.
In 1912 he made his Covent Garden debut as Cavaradossi in Tosca, and his Met debut in 1913 — the first of an eventual 663 performances at the Met. 1913 was also the year of the posthumous premiere of Massenet's Panurge, in which Martinelli sang Pantagruel. In 1915 he sang Lefebvre in the premiere of Giordano's Madame Sans-Gene, and in 1916, created the role of Fernando in Granados' Goyescas. Martinelli began to increase his repertoire to include most of the Italian and French dramatic roles — Canio, Otello, Samson, Radames, Manrico, Don Jose in Carmen, Alvaro in La Forza del Destino, Eleazar in La Juive, Vasco da Gama in L'Africaine, Arnold in Rossini's William Tell.
In the 1920s, Martinelli's home town of Montagnana built a theater to be named after their "two tenors" (the other being Aureliano Pertile). Much of Martinelli's career was focused in the United States, and it was not until 1937 that he returned to Covent Garden. In 1939, he sang Tristan und Isolde in Chicago with Kirsten Flagstad. In 1945 he stopped singing staged operas at the Met, but still participated in various benefit recitals. His last "full" role was as Samson in Philadelphia in 1950, but in 1967, he sang the Emperor in a Seattle production of Turandot.
Nimbus has re-released many of Martinelli's early HMV and Victor recordings (on NI 7804), which include arias from many of his most important roles and the famous excerpts from La Forza del Destino with di Luca, Ponselle, and Pinza. Pearl has released extended excerpts from complete opera productions that feature Martinelli, including a set with material from Simon Boccanegra and Otello.