The American soprano, Karan (sometimes written Karen) Armstrong was originally trained as a pianist. She graduated with a Bachelor of Music degree from Concordia College in 1963. She later studied with Lotte Lehmann in Santa Barbara, California.
Karan Armstrong made her operatic debut in 1965 with a minor company in San Francisco as Musetta in La bohème. She made her first appearances with the San Francisco Opera the following year as Elvira in L'Italiana in Algeri. In 1966 she won the Metropolitan Opera National Council Auditions which led to her being engaged by the house as a comprimario singer. In July 1966, she appeared at the Carmel Bach Festival under Sandor Salgo. She made her debut at the Met on October 2, 1966 as one of the servants in Richard Strauss's Die Frau ohne Schatten under conductor Karl Böhm with Leonie Rysanek as the Empress and Christa Ludwig as the Dyer's Wife. She continued to perform regularly at the Met up through the Spring of 1969 in smaller roles like the Dew Fairy in Hänsel und Gretel and Annina in La traviata. Better offers came her way from the New York City Opera, and she made her first appearance with that company as the Reine de Chémakhâ in Le coq d'or in 1969. She was to appear at that theatre many times through 1977, singing such roles as Violetta in La traviata (1975), the title heroine in La Belle Helene (1976), and Minnie in La fanciulla del West (1977) among others.
In 1974, Karan Armstrong first appeared in Europe, as Micaëla in Carmen, at the Opéra du Rhin in Strasbourg. The following year, she created a great sensation with her performance of Salome at the same theatre. Further performances in Europe followed, including Tosca in Venice, and Elsa in the 1979 Bayreuth Festival's Lohengrin, directed by her future husband, Götz Friedrich, which was later recorded and filmed. She also sang in Berlin (where she was to be a great favourite), Vienna, Paris, Covent Garden (Lulu, which Robert Craft once declared was "accurately sung and perfectly enacted"), Los Angeles, and the Bolshoi Theatre in Moscow.
Karan Armstrong has sung in several operatic world premieres, including Gottfried von Einem's Jesu Hochzeit (as Death), Giuseppe Sinopoli's Lou Salomé, Luciano Berio's Un re in ascolto, York Höller's Maître et Marguerite and Siegfried Matthus's Desdemona und ihre Schwestern. Other celebrated roles include those in Susannah, Salome, Les contes d'Hoffmann (as Giulietta), La voix humaine, Lohengrin, Pelléas et Mélisande, Lulu, Wozzeck, Der Rosenkavalier, Die tote Stadt, Parsifal, Erwartung, Die Walküre (as Sieglinde), Katya Kabanova, The Makropulos Case, Fidelio, Mathis der Maler (as Ursula), Tannhäuser (as Venus, with René Kollo) and Dialogues des Carmélites (as Mother Marie of the Incarnation).
In the 2009-2010 season (December/January), Karan Armstrong sang the Old Lady in Leonard Bernstein's Candide, at the Flanders Opera; Larina in Tchaikovsky’s Eugen Onegin, at the Deutsche Oper Berlin (March/April/May) as well as the Queen of Hearts in Unsunk Chin’s Alice in Wonderland at the Grand Teatre de Geneve (June).
In 1985, Karan Armstrong was named a Kammersängerin in Stuttgart; in 1994, she received the title in Berlin. She was married for many years to opera director Götz Friedrich. Their marriage ended upon Friedrich's death in 2000. The couple had one son together.