He first studied at the Manhattan School of Music—where he was a pupil of Friedrich Schorr and John Brownlee—and then at the Santa Cecilia Conservatory, Rome, with Luigi Ricci.
He quickly became a favorite with the audience in comic roles, such as Bartolo in The Barber of Seville and Dulcamara in Elisir d'amore, though he also excelled in more lyrical and dramatic repertory.
In his 27 seasons with the company, he sang, notably, Rodolfo in La sonnambula, Giorgio in I puritani, Raimondo in Lucia di Lammermoor, Silva in Ernani, Wurm in Luisa Miller, Sparafucile in Rigoletto, Fra Melitone in La forza del destino, Philippe II in Don Carlos, Pogner in Die Meistersinger von Nürnberg, Timur in Turandot, and other roles.
Flagello also enjoyed a successful international career, appearing frequently in Vienna, Milan, Berlin, London, and other places.
In addition to his operatic career, he had a small role in the flashback sequences in The Godfather Part II (1974) as an impresario threatened by Don Fanucci.