He published two novels and a number of novellas, but is mainly known for his many contributions as an art critic to the newspaper Dagbladet.
He was close friends with Edvard Munch,[1][2] Oda Krohg,[3] and Hans Jæger,[4] and he is considered one of the "Kristiania Bohemians.
Edvard Munch wrote about this unhappy love affair, and it also provided the background for his 1891 painting Melancholy.
Nilssen's first novel, Nemesis, describes the young writer Nils Falk, who has an unhappy love affair with a married woman ten years older than him.
In 1909, Jappe Nilssen and the director of the National Gallery, Jens Thiis, arranged an exhibition of 100 paintings and 200 sheets of graphics by Munch.