Born in Rome, Panni was the granddaughter of Giuseppe De Luca, and was trained as a singer at the Accademia di Santa Cecilia under Giannina Arangi Lombardi.
Her debut came in Trieste in 1957, when she sang Blanche in Dialogues of the Carmelites by Francis Poulenc.
She appeared abroad as well, singing at the Liceu and Teatro Nacional de São Carlos.
1962 saw her American debut, as Marguerite in Faust by Charles Gounod at the Philadelphia Opera.
She appeared with Lyric Opera of Chicago in 1964 as Zerlina in Don Giovanni and Micaela in Carmen, and in 1965 performed Leonora in Il trovatore for Baltimore.