In 1903, with financial support from the patron François Franck, Mortelmans founded the Maatschappij der Nieuwe Concerten ("Society of New Concerts") in Antwerp, which attracted notable guest conductors and artists such as Gustav Mahler, Siegfried Wagner, Hans Richter, Richard Strauss, Sergei Rachmaninoff, Pablo de Sarasate, Jacques Thibaud, Pablo Casals, and Fritz Kreisler.