Her sudden death at the age of 31 was initially attributed to epilepsy, but after the end of World War II, some commentators asserted that she was in fact murdered by Gestapo officers, others that she committed suicide.

She starred in more than twenty German films, including Viktor und Viktoria (1933), one of her biggest successes, which was remade decades later as Victor Victoria with Julie Andrews.

After making Sunshine Susie (1932) in England, she returned to Germany and was delayed by anti-German French officials for a short time in Paris.

With the rise of the Nazi Party, Müller came to be regarded as an ideal "Aryan" woman, and, particularly in light of Dietrich's move to Hollywood, was courted and promoted as Germany's leading film actress.

According to Channel 4 documentary "Sex and the Swastika", aired in February 2009, she jumped from a Berlin hospital window where she was being treated for a knee injury or drug addiction.