Nielsen grew up in Denmark and Germany, where she studied literature and eventually received the highest possible certificate or "abitur" in 1937 from a private school located in Flensburg.
Her professional work with psychoanalysis began at an anthroposophical clinic in the Swiss canton of Ticino, where she met her future husband Alexander Mitscherlich.
[5] In the 1950s, she completed her psychoanalytic training at the London institute led by Anna Freud, Melanie Klein and Michael Balint.
Along with Alexander Mitscherlich, she returned to Germany, taking up work at a psychosomatic clinic her husband directed at Heidelberg, before moving to Frankfurt.
Grundlagen kollektiven Verhaltens (The Inability to Mourn: Principles of Collective Behaviour), first published in 1967, discussing why The Holocaust, the war crimes, and the sentiment of guilt on the offender's part were not dealt with adequately in post-war German society.
[7] She is notable for the highly politicized nature of her work when many of her peers considered neutrality an essential element of psychoanalysis.