Thereafter he studied privately under Aksel Jørgensen before entering the painting school at the Royal Danish Academy of Fine Arts (1945–47).
During the course of the 1950s, he gained a reputation as one of Denmark's foremost graphic artists working with a wide variety of art forms including woodcuts, linocuts, etching, lithography, monotyping as well as with pen, paper and watercolours.
His striking urban and technological landscapes present a mixture of splendor and horror, exposing the root causes of war and violence.
[3] In 1950, he visited Hamburg which inspired his early Orfeus og Eurydike (Orpheus and Euridice) illustrations.
Shortly before his death, he summarized his philosophy as follows: "Man is good from the outset but then mismanagement sets in, and you become a scoundrel ... For if you obstruct existence, it will turn against you..."[4] From 1967 to 1973, Nielsen was a professor at the Royal Danish Academy of Fine Arts.