Born in Potsdam, Mersmann studies in Munich and Berlin.
In 1933, after the Nazi takeover, he was dismissed from the university on the grounds that he had worked in the field of Neue Musik.
He was then obliged to give private music lessons.
In 1935, he was still stigmatized as "Bolshevik of Music" by the Militant League for German Culture.
From 1947 to 1957, he taught at the Hochschule für Musik und Tanz Köln.