Opera News stated, "An ambitious beauty with a velvety, even-grained dramatic mezzo, Thebom was a natural for opera: she commanded the stage with the elegantly disciplined hauteur of an old-school diva, relishing the opportunity to play femmes du monde such as Marina in Boris Godunov, Herodias and Dalila.
"[3] While Thebom sang a wide repertoire which encompassed everything from Handel and Mozart to Verdi and Debussy, she was best known for her performances in the operas of Richard Wagner.
[4] She notably sang the latter role in the famous 1952 EMI recording made in London with Kirsten Flagstad, Ludwig Suthaus, and conductor Wilhelm Furtwängler.
[8] After Boghetti's death in July 1941, she studied with retired Metropolitan Opera mezzo-sopranos Edyth Walker and Margarete Matzenauer in New York City.
[3] 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.
She also appeared in two United States premieres at the Met; singing the roles of Baba the Turk in Igor Stravinsky's The Rake's Progress (1953) and Adelaide in Richard Strauss' Arabella (1955).
[3] Other roles she performed at the Met included Adalgisa in Norma, Azucena in Il trovatore, Dalila in Samson and Delilah, Eboli in Don Carlos, Geneviève in Pelléas et Mélisande, Giulietta in The Tales of Hoffmann, Herodias in Salome, Klytämnestra in Elektra, Laura Adorno in La Gioconda, Marfa in Khovanshchina, Marina in Boris Godunov (1956),[12] the Old Baroness in Vanessa, Orlofsky in Die Fledermaus, and the title roles in Carmen and Mignon.
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.
[10] In 1957 she came to London to sing Dido in the much lauded 1957 production of Hector Berlioz's Les Troyens at the Royal Opera House, Covent Garden.
In this production she made effective use of her spectacularly long hair, allowing it to fall down her back as she ascended the funeral pyre at the end.
[7] In 1960, she appeared at the Dallas Opera as Ruggiero in a celebrated production of George Frideric Handel's Alcina, with Joan Sutherland in the title role.
[3] While teaching at SFSU and later privately, Thebom served as chair of the Pacific Region Metropolitan Opera National Council Auditions for fifteen years.