After the
Machtergreifung by the
Nazis, Ambrosius became a member of the
NSDAP on 4 March 1933 under the party number 2,994,125.[2] Since 1936 he was also active as Gauobmann Mitte of the
Reichsmusikkammer.[2] From 1943 to 1945 he was a teacher at the "Städtische Musikschule für Jugend und Volk" in Leipzig. After he had been drafted into the
Wehrmacht for the first time in 1939, he was exempted from military service in 1940, but had to do military service again in 1944 until the end of the Second World War.[2] During the
National Socialism period he wrote various
cantatas and songs for male choir in conformity with the system in addition to symphonic and concertante music and the Deutschen Landschaftsbildern (1939).[3]
From 1945 Ambrosius worked as a private music teacher, choir leader and freelance artist. After his death, the city of
Engen honoured the composer and named a street after him.
Ambrosius left an extensive compositional legacy of over 500 works. Especially his compositions for
Zupfmusik [
de] are of importance. They had already been the focus of attention of soloists and
chamber music ensembles since the 1930s and have enjoyed ever increasing popularity ever since.[4] The "
Bund Deutscher Zupfmusiker [
de]" expressly promoted his work for this field and made him an honorary member.