From 1970 to 1980, Mandac assumed the feminine lead role (or one of the lead roles) in more than 20 operas staged at the Met, and has appeared in major opera houses not only in the US but also in Europe. Time Magazine once described her as “a specially beautiful singer” after Mandac sang with Marilyn Horne in Handel’s “Rinaldo”.
Her engagements have included international festivals such as England’s Glyndebourne, Austria’s Salzburg and Helburn, Italy’s Ravinia, etc. She has sung the operatic roles of Mimi in Puccini’s "La Boheme," Manon in Massenet’s "Manon," Princess Liu in Puccini’s "Turandot," Micaela in Bizet’s "Carmen," Melisande in Debussy’s "Pelleas et Melisande," Gilda in Verdi’s "Rigoletto," Leila in Bizet’s "Les Pecheures de Perles" and Inez in Meyerbeer’s "L’ Africana. "
Her lighter roles include Lauretta in Puccini’s Gianni Schicchi" (which marked her Metropolitan Opera debut in 1975), Zerlina in Mozart’s "Don Giovanni," Anna in Verdi’s "Falstaff," Norina in Donizetti’s "Don Pasquale," Sophie in Strauss’ "Der Zauberflote" and "Le Nozze di Figaro."
Conductors have eagerly accompanied her performances over television. Mandac recorded "Carmina Burana" with Seiji Ozawa and the Boston Symphony Orchestra, a performance that was named BEST IN INTERPRETATION by The Stereo Review. She sang in Tchaikovsky’s "Queen of Spades" with the famed Jennie Tourel, with Peter Adler conducting the BSO.
She has also performed with James Levine conducting the Cleveland Symphony, Jan Peerce, Brigit Nilsson, tenor Richard Tucker, Swedish baritone Hakan Hagegard, Placido Domingo, mezzo soprano Marilyn Horne, Shirley Barret, Marion Lippert, and Dame kiri Te Kanawa.