Outside of the Metropolitan Opera, Thebom had actively performed as a guest artist with opera companies throughout the United States and abroad. In 1946, she made her stage debut in Chicago as Brangäne with the
Chicago Opera Company. She made her debut with the
San Francisco Opera (SFO) the following year singing Amneris to the Aida of
Stella Roman. She was heard frequently in San Francisco through 1963; notably portraying the role of Mother Marie in the United States premiere of
Francis Poulenc's
Dialogues of the Carmelites at the SFO in 1957. Other roles she performed in San Francisco were Brangäne, Cherubino in
The Marriage of Figaro, Carmen, Dalila, Fricka, Giulietta, Laura Adorno, Marina, Octavian in
Der Rosenkavalier, and Orfeo in
Orfeo ed Euridice. She also sang Dalila to the Samson of
Giovanni Martinelli at the
Philadelphia Civic Grand Opera Company in 1950. Thebom made her European debut in 1950 as Dalila at the
Royal Swedish Opera (RSO). She returned to the RSO several times, singing such roles as Amneris, Eboli and — in what
The Times described as "a not especially successful attempt at a soprano role" — as Elisabeth in
Tannhäuser. In 1957, at the pinnacle of the
Cold War, Thebom became the first American to perform at the
Bolshoi Theatre in Moscow, where she portrayed
Carmen for 3 weeks. ==Post opera career==