Van den Nieuwelaar was best known for his modern, functional furniture designs, characterized by innovative use of materials such as aluminum and plastic. He also designed lighting fixtures, office accessories and objets d'art.
Van den Nieuwelaar won numerous awards for his creations, including the Prix de la Jeune Sculpture in 1966 and the Prix de l'Innovation en Design in 1970. His work has been exhibited in numerous design and museum shows around the world.
Over the years, Van den Nieuwelaar has also worked as a teacher and guest lecturer at various art and design schools in the Netherlands and abroad. His work continues to inspire contemporary designers and remains an example of the minimalist, functionalist approach of Dutch design.