In 1991, she made her debut at the Salzburg Festival singing 2 concerts in commemoration of Mozart's 200th death anniversary and as Annio in Mozart's La clemenza di Tito under Sir Colin Davis. In that same year she left Zurich to fulfill her contract at the Vienna State Opera debuting in the role of Rosina in Rossini's The Barber of Seville, and remained with the company for 2 years.

Her international career took flight in Salzburg in 1992 when at short notice, she stood in for Marilyn Horne as Tancredi in two concert performances of the Rossini opera. Since then she has been a regular guest at that summer festival, performing in Mozart's La clemenza di Tito, Idomeneo, Così fan tutte, Mitridate, the Mozart pastiche Ombra felice, and Berlioz's La damnation de Faust. She has also performed at other notable opera festivals such as Bregenz, St. Moritz, Glyndebourne, Munich and Pesaro.

Kasarova initially specialized in Mozart's operas and works by bel canto composers such as Rossini, Bellini, and Donizetti. By the mid 2000s she became much sought after in Baroque trouser roles such as Orphée in Gluck's Orphée et Eurydice, Ruggiero in Händel's Alcina, and the title role in Ariodante.