Marijnissen was responsible for the Conservation Department of the Royal Institute for Cultural Heritage in Brussels for 30 years. Furthermore, he was a member of the Belgian Royal Academy of Sciences, Lettres and Fine Arts.
He edited and co-edited numerous art publications. Marijnissen gained international recognition as a Bruegel and Bosch specialist. He was also an expert in art forgery and attached particular importance to technical art research.
In 2011, Marijnissen donated his personal library and archive to the Museum of Fine Arts in Ghent.
