A regular guest at the festivals of Salzburg, Glyndebourne and Lucerne, Haefliger was particularly praised for his interpretation of the Evangelist in Bach's Passions with the Munich Bach Choir and Orchestra under Karl Richter (with whom he recorded the works for DG Archiv). He was also noted for his performances of Schubert.

According to Grove, Haefliger's voice was "notable for its clarity and focus rather than its tonal quality, which tended towards the monochrome. His scrupulous attention to verbal articulation and his understanding of the niceties of phrasing were always evident in Bach and in song."

Born in 1919 in Davos, Switzerland, Haefliger studied singing and violin in Zurich; he later went to Vienna to study with the tenor Julius Patzak. He made his debut in 1949 at Salzburg, originating the role of Tiresias in Carl Orff's Oedipus. He also created roles in Blacher's Zwischenf‹lle bei einer Notlandung (1966, Hamburg) and Zweihunderttausend Taler (1969, Berlin), and in several operas by Frank Martin.

Haefliger first performed in North America at the 1959 Vancouver Festival, where he was heard by Bruno Walter, who invited him to record Mahler's Das Lied von der Erde.

He made his debut at the Lyric Opera of Chicago as Tamino in The Magic Flute and made numerous appearances at Avery Fisher Hall with the New York Philharmonic, as well as at the Metropolitan Museum and Carnegie Hall.

Haefliger enjoyed an equally successful recording career and won several awards for his recordings with labels including Columbia Records and Deutsche Grammophon, which included Mozart works and Beethoven's Fidelio. DG released a box set of twelve CDs called The Art of Ernst Haefliger, including Bach, Schubert song cycles and Janšcek.

He also taught for many years at the University of Music and Performing Arts in Munich.