His interest in art was influenced by his mother, who was a painter, and he maintained several contacts in the Norwegian art community.

Together with his brother, Haakon Onstad, he established the shipping company Niels Onstad Tankrederi A/S in 1935. In 1940, Onstad moved to the U.S. where he worked for Nortraship. During WWII, this company managed the large Norwegian merchant fleet outside of the German-controlled waterways and contributed decisively to the Allied efforts during the war.

Niels Onstad and Sonja Henie met one another in the U.S.A. in 1955. His frequently quoted comment, "I prefer tomorrow's art," was indicative of the focus of their mutual art collection.