He made his debut in his native Livorno, as Cavaradossi in Giacomo Puccini's Tosca in 1923.
He sang as first tenor at the Rome Opera from 1930 to 1950, and was also a regular guest artist at La Scala, Milan, and the Verona Arena.
Masini was particularly acclaimed in roles such as Alvaro, Radames, Loris and Calaf, although he also enjoyed success in more lyrical parts, notably as Edgardo, Rodolfo and Pinkerton.
Masini was not regarded by music critics as being a particularly subtle stylist as a singer; but the impressive quality of his vocal instrument was unquestioned.
His large, ardent and ringing voice can be heard to best advantage in a complete recording of Verdi's La forza del destino, under the baton of Gino Marinuzzi, that was made by Cetra Records in 1941.