Gigli was born in Recanati near Ancona, the son of a shoe-maker.
In 1914, Gigli won first prize in an international singing competition in Parma. His operatic debut came on October 15, 1914 when he played Enzo in Amilcare Ponchielli's La Gioconda in Rovigo, following which he was in great demand. The following year he sang Faust in Arrigo Boito's Mefistofele in Naples, a role he would sing many more times.
Gigli made his debut at the Metropolitan Opera in New York City on November 26, 1920, again in Mefistofele. Other roles he became particularly associated with included Rodolfo in Giacomo Puccini's La Bohème and the title role in Umberto Giordano's Andrea Chénier, which he also recorded in full.
Gigli left the Met in 1932 after refusing to take a pay cut. He returned again to Italy, and sang in houses there, elsewhere in Europe and in South America. He was criticized for being a favourite singer of Benito Mussolini, but this did him no lasting damage, and, after retiring from public performance in World War II, he returned successfully to performing in 1945.
In the last few years of his life, Gigli gave concert performances more often than he appeared on stage. He died in Rome in 1957.