[2] As a politician, Müller, who belonged to no party,[5] served as Federal Minister for Economic Affairs and Energy from 1998 to 2002, when Gerhard Schröder (SPD) was Chancellor.

[2][5] After the 1998 German federal election, Schröder made him minister, after the designated candidate, Jost Stollmann had turned down the position because the responsibilities of the ministry were being reduced.

[1][5] According to former employer Evonik and German President Horst Köhler, Müller managed a reduction in Germany's dependence on coal (Ausstieg aus der Kohleenergie) as an energy source, trying to mitigate the issues caused by this fuel, namely damage to the environment and the large subsidies needed in mining,[3] in a socially responsible way.

[11] In April 2018, he received the Order of Merit of North Rhine-Westphalia for his life's achievements (für sein Lebenswerk) in the presence of Gerhard Schröder.

[8] After his death, Schröder acknowledged Müller as a great economic leader who knew the rules of the game of both business and politics, which enabled him to turn opponents to partners.

Jean-Claude Juncker, President of the European Commission, noted Müller's enormous expertise, dry humour and calm manner, achieving invaluable benefits for the German miners.

Armin Laschet (CDU, Minister-president of North Rhine-Westphalia, called him a visionary, and the founding of the RAG-Stiftung "a genuine work of the century" (ein echtes Jahrhundertwerk).