 Mayer received the Iron Cross 2nd Class () on 25 October 1939 and was transferred to "Jagdgeschwader" 2 "Richthofen" (JG 2—2nd Fighter Wing), named after the after World War I fighter ace Manfred von Richthofen, on 6 December 1939.For his entire combat career, with the exception of a brief posting to the fighter pilot school at Werneuchen, Mayer served in JG 2 "Richthofen".He claimed his first aerial victory on 13 June 1940 during the Battle of France, shooting down a Morane-Saulnier M.S.406 belonging to the French Air Force ("Armée de l'Air").