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Mandac has premiered new works including world and American premieres of Thomas Pasatieri's Black Widow and Inez de Castro, Luciano Berio's Passagio, Hans Werner Henze's Bassarids, and Richard Bennett's Mines of Sulphur.
Mandac came to the United States on a Fulbright Scholarship and studied at Oberlin Conservatory of Music. She then studied at the Juilliard School of Music on a Rockefeller Grant, where she received her master's degree. She won the Metropolitan Opera Auditions (1966)  and the Queen Elizabeth International Voice Competition in Belgium.
Following her tenure with the Lindemann Program at the Metropolitan Opera, she went on to perform with opera companies all over the world including San Francisco Opera where she starred as Susanna in Le nozze di Figaro, Despina in Cosi fan Tutte (with Frederica von Stade and Evelyn Lear), and Ines in L'Africaine (with Shirley Verret and Plácido Domingo).
- Carl Orff's Carmina Burana with the Boston Symphony Orchestra conducted by Seiji Ozawa under the RCA Label.
- Giacomo Meyerbeer's L'Africaine at the San Francisco Opera with Plácido Domingo and Shirley Verrett.
- Gustav Mahler's Symphony No. 2 with the Philadelphia Orchestra conducted by Eugene Ormandy.
- Josef Alexander's Songs for Eve with the Philadelphia Orchestra under the Seraphim label.