Martinelli was born in Montagnana, Veneto.
After service as a clarinetist in a military band, he studied with Giuseppe Mandolini in Milan, and made his professional debut at the Teatro Dal Verme, as Ernani in 1910. The role of Dick Johnson in La fanciulla del West became his passport role, he sang it for his debut in Rome (under Toscanini), Brescia, Naples, Genoa, all in 1911, as well as in Monte Carlo and La Scala, in 1912. Cavaradossi in Tosca, was his debut role at the Royal Opera House, Covent Garden, in London, and for his first American engagement in Philadelphia, in 1913. On 25 April 1913 he portrayed Pantagruel in the world premiere of Jules Massenet’s Panurge at the Théâtre de la Gaîté in Paris.
Martinelli’s New York Metropolitan Opera debut took place on November 20, 1913, as Rodolfo in La Bohème, where the young tenor’s easy high C and pure, silvery tone attracted favorable attention. He remained a Met mainstay for 32 seasons, with 926 performances of 36 roles. He appeared most often as Radames in Aida, Otello, Manrico in Il trovatore, Don Alvaro in La forza del destino, Calaf in Turandot, and Dick Johnson in La fanciulla del West, but also as Arnold in Guglielmo Tell, Eleazar in La Juive, Enzo in La Gioconda, Don Jose in Carmen, Vasco de Gama in L’Africaine, Canio in Pagliacci, Pollione in Norma.
Martinelli also sang in Boston, San Francisco and Chicago, often trying out new roles before singing them at the Met.