Influenced by the works of Wassily Kandinsky, he developed an abstract art style.
In Copenhagen, Mortensen was joint founder of the "Linien" school of abstract painters.
[3] In 1937, he undertook a study trip to Paris, where he met pioneers of surrealism, such as Salvador Dalí, Roger Vitrac, Gala Éluard, Michel Leiris, Antonin Artaud, Raymond Queneau and André Masson.
Together with Robert Jacobsen, Mortensen became connected to the Galerie Denise René in Paris, which became famous for concrete art.
After his return to Denmark in 1964, he received a professorship at the Royal Danish Academy of Art in Copenhagen, which he held until 1980.