Koos Wassenaar was educated at the academy in The Hague (1956-1963), department of graphic and typographic design. As a painter he initially made still lifes, figures and abstracted landscapes. After 1965 he almost exclusively painted figures in a strongly reduced form. In addition, the artist was engaged in the manufacture of ceramics and graphics. From 1960, Koos Wassenaar was a lecturer at the T.H. Delft and at the Academy for Visual Education in Tilburg.