[1] She played minor roles as part of her family's engagement with the short-lived Opera Comique (1918–1921), and had her serious stage debut (1926) as Hannah in Eduard Künneke's The Cousin from Nowhere.
Her first major roles were as Amor in Christoph Willibald Gluck's Orfeo ed Euridice (Central Theater, 1933) and Giuiletta in Jacques Offenbach's The Tales of Hoffmann (National Theatre, 1938) under the direction of Olav Kielland.
She was married to the Austrian pianist Johann Crkal (1891–1939)[4][Note 1] and she performed in Italy and France as Karen Maria Crkal,[6][7][8] first as a guest together with her sister as Brünnhilde at La Scala as Ortlinde in Wagner's Die Walküre (1949).
[9] She also appeared in other operas (including Carl Maria von Weber's Der Freischütz, Beethoven's Fidelio, and Eugen d'Albert's Tiefland for NRK), appeared on radio with her brother Lasse Flagstad on piano (1946), was a director in the Norwegian Opera Association (Norsk Operaforbund), and worked with the People's Theater in Oslo.
Flagstad was employed at the Norwegian National Opera and Ballet (led by her sister) from 1958 to 1961, where she was acclaimed in the role of Ågot in Waldemar Thrane's Fjeldeventyret (The Mountain Tale, 1960) and later worked as a répétiteur (like her mother).