He specialized in music of the Baroque period, but later extended his repertoire to include Classical and early Romantic works.

Starting out as a classical cellist, he founded his own period instrument ensemble, Concentus Musicus Wien, in 1953, and became a pioneer of the Early Music movement.

His father, Eberhard Harnoncourt, born de la Fontaine Graf d'Harnoncourt-Unverzagt,[3] was an Austrian engineer working in Berlin who had two children from a previous marriage.

Two years after Nikolaus's birth, his brother Philipp was born, and also in 1931, the family moved to the Styrian capital Graz, Austria, where they took up residence in their ancestral home, Palais Meran.

He also made the first recordings in historically informed performance of Bach's Mass in B minor (1968) and St Matthew Passion (1970).

In 2001, an acclaimed and Grammy Award winning recording of the St Matthew Passion with the Arnold Schoenberg Choir was released, which included the entire score of the piece in Bach's own hand on a CD-ROM.

[19] In October 2000, the Royal Concertgebouw Orchestra (KCO) named him their Honorair gastdirigent (Honorary Guest Conductor).

An accompanying second CD contained a lecture by Harnoncourt about the symphony with musical examples, including the rarely heard fragments from the unfinished finale.

"My bodily strength requires me to cancel my future plans," he wrote in a hand-written letter inserted into the program on his 86th birthday of a concert by the Concentus Musicus Wien.

[20] Harnoncourt was the focus of the annual festival of classical music Styriarte, founded in 1985 to tie him closer to his hometown, Graz.

[21] Harnoncourt met his wife Alice through their mutual interest in historically informed performances of Baroque music and co-founded the Concentus Musicus Wien.

[22] Harnoncourt died on 5 March 2016 in the village of Sankt Georgen im Attergau, north east of Salzburg.

Harnoncourt was a member of the Royal Swedish Academy of Music[31] and of the Order Pour le Mérite for Science and Art,[31] and an Honorary Doctor of the University of Edinburgh.