Since 1985 Althaus has been a member of the CDU, remaining with the party as it transformed itself from a loyal supporter of the GDR's ruling Socialist Unity Party of Germany (SED) to a loyal supporter of the West German party of the same name with which it merged in 1990 shortly after German reunification. In 2000 he became chairman of the CDU in Thuringia. Since 1990 he has been a member of the Thuringian Landtag.
In 1992 he became a member of Bernhard Vogel's cabinet as minister of culture and education.
On 5 June 2003 he was elected Minister-president of Thuringia; he succeeded Bernhard Vogel, who had resigned for reasons of age. As Minister-President he served as President of the Bundesrat in 2003/04.
Althaus is married to Katharina and has two children. He caused a skiing collision in Styria, Austria on 1 January 2009 in which he suffered severe injuries. Althaus was skiing down an expert run, but wandered onto an easy slope, where he was skiing in the wrong direction, whereupon he and a 41-year-old Slovak woman collided. The woman subsequently died from her injuries. Althaus was wearing a skiing helmet, while the woman was not. Althaus was fined €33,300 for negligent homicide.
After the Thuringia elections of 2009, where the CDU went from having an absolute majority to not even having enough seats to form a majority coalition with the FDP, Althaus resigned as minister-president and as chairman of the CDU in Thuringia.
- "German politician Dieter Althaus guilty of manslaughter in Austrian ski death German politician Dieter Althaus has been found guilty of Tuesday of involuntary manslaughter for his role in a fatal skiing accident and was fined 33,000 euros (£29,500) by an Austrian court.". The Daily Telegraph (London). 2009-03-03. Retrieved 2010-05-01.
- "German State Premier Seriously Injured in Skiing Accident". Deutsche Welle. Retrieved 2009-01-02.
- "German charged over ski death". BBC News. 2009-03-02. Retrieved 2010-05-01.
- Smith, Nicola (2009-01-18). "Merkel ally may face trial over fatal ski crash". The Times (London). Retrieved 2010-05-01.