She was recognised internationally when she appeared at the Glyndebourne Festival in 1965 in the title role of Der Rosenkavalier, alongside Montserrat Caballé, and two years later as Donna Elvira in Mozart's Don Giovanni.

From 1970, she was a long-time member of the Metropolitan Opera in New York City, where she appeared in leading roles including Desdemona in Verdi's Otello, and Puccini's Mimì, Liù and Manon Lescaut.

By now fluent in German, she appeared as Rosalinde in Die Fledermaus by Johann Strauss, which requires spoken dialogue, and as Agathe in Weber's Der Freischütz.

[1] During her residence in Germany, Żylis-Gara was active internationally, beginning in 1965 at the Glyndebourne Festival as Rosenkavalier, with Montserrat Caballé as Feldmarschallin, and in 1967 as Donna Elvira in Mozart's Don Giovanni.

[1][28] Irving Kolodin of the Saturday Review wrote of her performance: Zylis-Gara, who has emerged from her Polish background onto the international operatic stage within the last eight or so years, has unusual attributes for Elvira.

Her voice is both bright and substantial, which means that she can convey the intensity of emotion in the role – she is, after all, the only one in the cast who truly loves the Don – without sounding shrewish.

[8]After this performance, Rudolf Bing, general manager of the house, offered her a long-term contract with the company beginning in January 1970 with Pamina in Mozart's The Magic Flute.

She remained on the Met roster for the next 14 seasons, portraying such roles as Countess Almaviva in Mozart's The Marriage of Figaro, Amelia in Verdi's Un ballo in maschera, Madame Butterfly,[29] Desdemona in Verdi's Otello, Elisabeth in Wagner's Tannhäuser, Elsa in Lohengrin, Fiordiligi, Leonora in Verdi's Il trovatore, Liù in Puccini's Turandot, Marguerite in Gounod's Faust, Mimì in Puccini's La bohème, Octavian in Der Rosenkavalier, Tatiana in Tchaikovsky's Eugene Onegin, La traviata, and the title roles in Cilea's Adriana Lecouvreur, and Puccini's Suor Angelica and Tosca.

She took part in the Gala honoring Rudolf Bing, as Desdemona in the love duet from Otello with Franco Corelli, conducted by Karl Böhm.

In her performances at the Met, numerous artists made their house debuts, including Hildegard Behrens, Neil Shicoff, Kurt Moll, Isola Jones, and Siegfried Jerusalem.

A reviewer of Turandot at the Aix-en-Provence Festival wrote that she was "a Liù full of sweetness, whose lovely pianissimi would overcome a heart even colder than that of the cruel princess".

In 1966 she recorded Bach cantatas with the Windsbacher Knabenchor, conducted by Hans Thamm, including Wer nur den lieben Gott läßt walten, BWV 93.

[41] A performance of Donizetti's Anna Bolena, broadcast from Cologne by Westdeutscher Rundfunk (WDR) in 1967, featured Żylis-Gara in the title role, Karl Ridderbusch (Enrico), Vera Little (Giovanna), Gene Ferguson (Percy), Wolfgang Anheisser (Rochefort), Barbara Scherler (Smeton) and Werner Hollweg (Hervey), was conducted by Alberto Erede, and has been reissued on CD.

[42] In 1986, she recorded live Bach cantatas for soprano solo including Mein Herze schwimmt im Blut, BWV 199, at the inauguration of the Théâtre La Colonne in Miramas, conducted by Dominique Debart.