Hahnemann was primarily active in touring car racing. In the 1980s and 1990s, he drove - initially primarily for Ford, later for BMW - over 100 races in the DTM.
For long distance races he reached inter alia an overall victory in 1991 at the 24 -hour race at the Nürburgring together with Joachim Winkelhock and Kris Nissen on a BMW M3 and a fifth place in the 24- hour race at Le Mans in a Porsche 911 GT1 in 1997. the following year he went also to Porsche 911 GT1 in the FIA GT Championship; together with Michael Bartels he reached it a podium finish in the second race of the season.
Since 2007, Hahnemann starts for Manthey Racing in the VLN Endurance Championship Nürburgring. In 2008 he finished in Porsche 996 GT3 - MR, the winning car from 2006, the 24 -hour race at the Nürburgring in second place. 2009 Hahnemann denied the race together with André Lotterer and others on a Lexus LF- A, but did not come in the final scoring.