Nissen was born in
Haderslev,
Denmark–Norway. He completed his schooling in 1781 and became "authorized agent of the General Post Office" in Copenhagen in 1781. In 1792 he became a diplomat in the Danish foreign service. As of 1793, he worked in Vienna as . In 1797, while serving in this post, Nissen first met Mozart's widow,
Constanze, whose husband had died six years earlier in 1791. He was initially her tenant. The two began living together in September 1798. Constanze had been through an arduous period following Mozart's death, trying to ward off poverty for herself and her two sons. In this she was successful, obtaining a pension from the Emperor, and making considerable money from concerts of Mozart's music and sale to publishers of his works in manuscript. Nissen came to participate in these labours, taking over much of the work of negotiating with publishers. He also helped care for the children, eventually taking (in Ruth Halliwell's words) "the role of a caring father" in the family. Nissen and Constanze were married 26 June 1809. The wedding took place in
St Martin's Cathedral, Pressburg (today
Bratislava), to which the foreign diplomatic corps had temporarily decamped when
Napoleon's armies
took Vienna. The marriage did not produce any children. In 1812 the couple moved to Copenhagen, where Nissen took up a post as a censor. They lived there until 1820, at
Lavendelstræde 1, a street where many houses of the period are still preserved. In 1820, Nissen retired, and the couple moved to Salzburg. Nissen had long been planning a biography of Mozart, and the work began seriously in 1823. The completion of the work, based on his notes, was left to a medical doctor and Mozart enthusiast living in
Pirna, Johann Heinrich Feuerstein (1797–1850).
Angermüller and Stafford call Feuerstein "unstable" and Halliwell judges his work thus: "the book was cobbled together in a haphazard fashion from the raw material, and the result was disastrous in terms of quality." Nissen died, aged 62, in Salzburg, and was buried there. His tombstone, naming him "The husband of Mozart's widow", can still be visited. ==Nissen's biography of Mozart==