[6] In June 1929 he challenged for Michele Bonaglia's IBU European light heavyweight title in Turin, the Italian stopping him in the fourth round.

[2] Müller moved up to heavyweight, and after a draw with German (VDF) champion Hans Schoenrath in a non-title fight in November 1930, faced him with the title at stake in February 1931.

[10] He suffered only the second defeat of his professional career in April 1932, when he was beaten on points by McCorkindale at the Royal Albert Hall, McCorkindale a substitute for Gains who had injured his thumb (the agreed fight with Gains required the lifting of the colour bar that had been in place at the venue since 1923),[10][11] and in his next fight, in May, lost his European title, with Charles taking a points verdict to regain his title.

[12] Müller's career never recovered, winning only two of his last nine fights (over Hans Schoenrath and Gustave Limousin), suffering knockouts at the hands of von Porat and Jack Petersen (in front of 50,000 spectators at Ninian Park), and failing in three attempts to take the BDB German heavyweight title from Vincenz Hower.

[16][17] During World War II, Müller was initially exempted from military service due to an eye condition, but in March 1945 he was enrolled into the Volkssturm.