A Berlin native, Anja Silja began with vocal studies already at the age of six. At the age of 10, she gave her first concert at the Titania-Palast in Berlin.

At the age of 16, she started her career on stage in Braunschweig with roles like Rosina (Barbiere di Siviglia), Zerbinetta (Ariadne auf Naxos) and Micaëla (Carmen). In 1958, engagements by the State Opera Stuttgart and Frankfurt Opera followed, and in 1960 Wieland Wagner brought her to Bayreuth, where she debuted with Senta (Der fliegende Holländer). Until Wieland Wagner’s death she sang in almost all of his productions in Bayreuth. Throughout Europe, she was known for her interpretations in the most important Wagnerian roles such as Isolde, Brünnhilde, Eva, Elisabeth, Elsa, Venus. Her repertoire also included roles from Königin der Nacht, Fiordiligi, Konstanze, Santuzza, Leonore ("Fidelio" and "La forza del destino"), Desdemona, Lady Macbeth, Lulu and Marie (Wozzeck).

In recent seasons, Anja Silja performed in Vienna, Zürich, Barcelona, Berlin, Hamburg, New York, London, Paris, Aix-en-Provence and at the Glyndebourne Festival Opera mainly in the great female characters of Leos Janacek like Kostelnicka (Jenufa) and Emilia Marty (The Makropulos Case).
She won a Grammy Award for her interpretation of Kostelnicka. Her repertoire also included Ortrud (Lohengrin), Amme (Die Frau ohne Schatten) and Klytämnestra (Elektra), as well as Mère Marie and Madame de Croissy (Dialogues des Carmélites), which she has sung at La Scala Milan under the baton of Riccardo Muti. 

Numerous opera recordings with the famous conductors of the 20th century prove evidence of her artistic work, a.o. "Der fliegende Holländer", "Lohengrin", "Tannhäuser", "Lulu" and "Wozzeck" by Berg (both for DECCA), "Salome", Fricka in Wagner’s "Ring", "Erwartung" by Schönberg, "The Makropolus Case" and "Jenufa" by Janacek (on  DVD) as well as the most important works of Kurt Weill.

For her great contribution to the field of art and culture, Anja Silja was honoured with the Order of the Federal Republic of Germany (Bundesverdienstkreuz).