After Hitler's appointment as chancellor on 30 January 1933, Müller served from 1933 to 1935 as head of the construction of the mobilization system in the General Staff of the Military District Command VII, in Munich.

During his time as a POW, Müller had an apparent change of views and professed to have become an anti-Nazi: within days of his capture he had joined the National Committee for a Free Germany and the Bund Deutscher Offiziere led by Walther von Seydlitz-Kurzbach.

He was one of a group of generals captured during Operation Bagration (including Edmund Hoffmeister, the commander of XXXXI Panzer Corps, and Rudolf Bamler of the 12th Infantry Division) who became especially prominent in NKFD activity.

Along with Bamler, Müller is known to have attended special training in Krasnogorsk late in 1944, and is believed to have been recruited by the Soviet secret services to spy on fellow NKFD members, such as Friedrich Paulus.

After heading the Ministry of the Interior, and successfully developing the Volkspolizei, he was appointed the Chief of Staff of the newly formed National People's Army - effectively the second-in-command of the East German military behind Willi Stoph.

[1] He died in 1961, in somewhat controversial circumstances, as he fell from the balcony of his home on the day he was scheduled to return to hospital; it was rumoured that he had committed suicide when a police vehicle drew up outside.

A posthumous autobiography, Ich fand das wahre Vaterland (English: I found the true fatherland), was published in 1963, edited by Klaus Mammach, an SED historian.