Alagna was born in Clichy-sous-bois, Paris, France. His family of Sicilian immigrants was very musically talented and young Alagna began singing pop in Parisian cabarets as a teenager. Influenced primarily by the films of Mario Lanza, but also from recordings of many historic tenors, he then switched to opera. He is largely self-taught as a singer, but he learned most of the tenor repertory from Cuban contrabassist and opera fan Rafael Ruiz.
After winning the Luciano Pavarotti Voice Competition, Alagna made his professional debut in 1988 as Alfredo Germont in La Traviata with the Glyndebourne touring company. This led to many engagements throughout the smaller cities in France and Italy, mainly again as Alfredo, a role he would eventually sing over 150 times. His reputation grew and he was soon invited to sing at major theaters such as La Scala (again as Alfredo, under the baton of Riccardo Muti) in 1990, Covent Garden in 1992 and the Metropolitan Opera as Rodolfo in La Bohème. His performances of Romeo in Romeo et Juliette by Charles Gounod at Covent Garden in 1994 catapulted him to international stardom.
A superb and dynamic stage actor, Alagna has a particular affinity for French opera and more lyric Italian roles, although some observers are worried about his taking on of progressively heavier Italian repertory, such as the title role in Otello. He is also devoted to unearthing French and Italian operatic rarities and showing them to a new audience.
Alagna's first wife, Florence Lancien, died of a brain tumor in 1994. He is now married to soprano Angela Gheorghiu. The couple have sung together often onstage and have made many recordings together both of duets and arias and complete operas.