[12] Mandac performed in five American premieres: two at Juilliard (Jenny in The Mines of Sulphur by Rodney Bennett and She in Passaggio by Luciano Berio in New York which she then performed in Turino, Italy with the RAI Orchestra) and three professional ones (Sicle in L'Ormindo by Francesco Cavalli in Washington National Opera, Autonoe in Bassarids by Hans Werner Henze at Santa Fe Opera, and the first staged US production of Handel's Almirea in Rinaldo with Marilyn Horne).
She has also performed under the batons of major names in opera including John Pritchard, Peter Maag, Jean Périsson, Emerson Buckley, Sixten Ehrling, Lawrence Foster, Herbert von Karajan, James Levine, Claudio Abbado, and Zubin Mehta and shared the stage with the likes of Sherill Milnes, Richard Tucker, Jenny Tourel, Frederica von Stade, James Morris, Kiri te Kanawa, Shirley Verrett, Placido Domingo, Marilyn Horne, and Birgit Nilsson .
She has been directed by notable stage directors including Jean Pierre Ponnelle, Ian Strasfogel, Bodo Igesz, Christopher West (of Covent Garden), Tito Copabiano, Frans Boerlage, Frank Corsaro, Bliss Hebert, Fabrizio Melano, Peter Adler, Lotfi Mansouri, Glynn Ross, Gian Carlo Menotti, and a Peter Hall production.
She is a firm believer that vocal technique serves one's dramatic and artistic impulses in music, and feeds the creative energy to express which is evident in the performances of her students, past and present.
Mandac played the role of "Lisa" in Peter Herman Adler's 90-minute, English-language version of Tchaikovsky's Queen of Spades produced for National Educational Television, which aired on February 28, 1971.