He first was taught by Olga Glomsås, Carl Finberg, Thomas and Nils Larsen in Oslo and continued with Edwin Fischer in Berlin and Santiago Riera in Paris. He was active as a performer and teacher for piano.
An article on Wikipedia mentions that Johnsen became a member in the so-called “Departementets midlertidige consultative råd i kunstneriske spørsmål” (“the Department’s Temporary Advisory Board for Artistic Questions”) in 1940, a NS-loyal institution, though crediting no source for this argument.
