In January 1945, Müller was forced by the Nazis to be part of a “Todesmarsch” (death march) to Mauthausen camp. He was then transported to Gunskirchen subcamp nearby, from where he was liberated by the Allies in early May 1945.
Later Müller gave testimony of his experiences to The Wiener Library in the 1950s, at the second Auschwitz trial in Frankfurt/Main in 1964 and to Claude Lanzmann for his documentary film Shoah (1985). He published a memoir of his time in Auschwitz in 1979. In the 1960s, Müller lived in Prague for some time. He died at the age of 91 in 2013.