After graduating from RWTH Aachen University with a doctorate in mechanical engineering, Theissen joined BMW in 1977. He became BMW Motorsport Director in April 1999 with oversight of BMW's Formula One team and other motorsport activities, including BMW's factory entries in the World Touring Car Championship and the 24 Hours of Le Mans.
BMW entered Formula One with a partnership agreement with WilliamsF1 signed in 1998. In 2001, BMW was credited with having the most powerful engine on the grid. While achieving notable successes, such as a strong championship challenge in 2003, the relationship between team and engine maker began to deteriorate. During the course of the 2004 and 2005 F1 seasons, Theissen increasingly became publicly critical of the WilliamsF1 team's inability to create a package capable of winning the constructors' championship, or even multiple victories within a single season. However WilliamsF1 blamed BMW for not producing a good enough engine. This public deterioration of the relationship between BMW and WilliamsF1, and between Theissen and Patrick Head in particular, was a factor in BMW's decision to buy Sauber.
Theissen was made team principal of the resulting BMW Sauber team, and in their first season, they achieved 5th in the Constructors championship, both a considerable improvement on Sauber's previous season and a match for Williams' last position in the standings, achieved in colloboration with the marque.
In 2007 the BMW Sauber team finished 2nd in the World Constructors Championship after McLaren-Mercedes were excluded following a spying episode.
In the 2008 Canadian Grand Prix Robert Kubica led Nick Heidfeld to finish 1 and 2 for the team giving Dr. Eng. Theissen his first victory for BMW as a constructor.