Thebom's first prominent engagement as a performer came in November 1941 when she made her first appearance as a soloist with the Philadelphia Orchestra under conductor Eugene Ormandy with fellow guest the University of Pennsylvania Glee Club at the Academy of Music in Philadelphia [REF].
In 1951 Thebom appeared as Dorabella in the premiere of Alfred Lunt's popular English-language production of Wolfgang Amadeus Mozart's Così fan tutte at the Met.