Around the age of 14, Nieuwenhuys and his two-year older brother decided to become painters.
During World War II Nieuwenhuys painted almost only clowns, nudes and couples making love.
Just after the war he started painting fantasy animals like aggressive cocks, cats and bulls.
During the war Constant himself painted only Catholic scenes like Pietà and Maria portraits or still lifes and thought that Nieuwenhuys chose his subjects too lightly.
In 1948 Appel, Elburg, Kouwenaar, Wolvekamp, Corneille, Constant Nieuwenhuys, Brands, Rooskens and Nieuwenhuys founded the Experimentele Groep in Holland that a few months later became the European group CoBrA.
Jan was in his paintings of this period influenced by dreams, children's drawings, the artistic expressions of mentally handicapped people and primitive art.
Nieuwenhuys was soon disappointed in the members of the CoBrA group, some of them were more interested in fame than being an activist.
Some other members also left, but they later rejoined for the great exhibition in the Stedelijk Museum.
We wanted to put an effort into that, to fight against the softness in art at the time and using our imagination to change that.