Bernhard began racing in karting in 1991 and progressed through junior championships, finishing fifth at the CIK/FIA Junior World Championship and winning the German junior karting title. He moved into formula cars in 1998, competing in Formula Ford in both the German series and the Eurocup, and finishing third in the German Formula Ford championship in 1999. Those seasons in open-wheel racing formed the foundation for what would become an exclusively sportscar-focused professional career.
In 2000, Bernhard joined the Porsche Supercup as a factory junior driver, finishing third in the championship. His American Le Mans Series debut came in 2001 at the 12 Hours of Sebring, where he shared a drive with Randy Pobst and Christian Menzel, taking a class podium. He also won the Porsche Carrera Cup Germany championship that year.
In 2002, Bernhard claimed a GT class win at the 24 Hours of Le Mans alongside Kevin Buckler and Lucas Luhr. The following year, 2003, brought his breakthrough overall victory at the 24 Hours of Daytona — driving a Porsche 911 GT3-RS with Buckler, Michael Schrom, and Jörg Bergmeister. That partnership with Bergmeister continued through 2004, when the pair took six wins from nine ALMS starts and secured the GT class driver championship, including class victories at Sebring and Petit Le Mans.
From 2005, Bernhard increasingly partnered with Romain Dumas. The pair moved into the LMP2 class in 2006 driving the Porsche RS Spyder for Penske Racing. At Mid-Ohio that year, Bernhard and Dumas scored an outright overall victory — the first-ever overall win for an LMP2-class car and the first under-class car to win outright since 2003. Bernhard won the ALMS LMP2 class championship in both 2007 and 2008, adding victories at the 24 Hours Nürburgring in 2006, 2007, 2008, and 2009. The 2008 season also delivered the 12 Hours of Sebring overall win alongside Dumas and Emmanuel Collard, completing two legs of the triple crown.
For 2009 and 2010, Bernhard and Dumas moved to Joest Racing, running Audi machinery. At the 2010 24 Hours of Le Mans, Bernhard, Dumas, and Mike Rockenfeller won outright in the No. 9 Audi R15, covering 397 laps and 5,410 km — breaking the 1971 distance record. The result completed Bernhard's triple crown of endurance racing and made him only the eleventh driver in history to achieve the feat. He added a second Le Mans overall victory in 2017.
On 29 June 2018, Bernhard drove a derestricted Porsche 919 Evo at the Nürburgring Nordschleife and set a new all-time lap record of 5:19.546, surpassing Stefan Bellof's 1983 benchmark set in a Porsche 956. It was the first time in 35 years the outright record had been broken, and the achievement underscored both Bernhard's status as a test and development driver of the highest calibre and Porsche's engineering dominance in that era.
Bernhard's career titles and class victories include the ALMS LMP2 class championship (2007, 2008), the ALMS GT class championship (2004), Porsche Carrera Cup Germany championship (2001), overall victories at the 24 Hours of Le Mans (2010, 2017), 24 Hours of Daytona (2003), 12 Hours of Sebring (2008), and 24 Hours Nürburgring (2006, 2007, 2008, 2009, 2011). His career stands as one of the most complete in German sportscar racing history.