Carosio, born in Genoa to a teacher/composer father, began singing in public at 14 and made her operatic debut at 16 as Lucia di Lammermoor at Novi Ligure. Soon after, in 1928, she made her Covent Garden debut as Feodor in Boris Gudnov, singing in Italian while bass Feodor Chaliapin sang in Russian and the chorus sang in French.
She made her La Scala debut as Oscar in Verdi's Ballo in maschera, followed by Philine in Thomas's Mignon.
Until Maria Callas came on the scene, Carosio was the leading bel canto soprano in Italy, famous for her performances in light lyrical roles such as MimÐ and Violetta. One of the turning points of Callas' career came when she replaced an indisposed Carosio as Elvira in I puritani at La Fenice in 1949.
She sang lead roles into her early 60s, and retired from singing in the mid-1950s. Afterward, she devoted herself to music criticism and journalism.