Maassen began racing in karts before transitioning to cars in 1989, starting in Formula Ford 1600. During the early 1990s he competed in German Formula 3, where he claimed his most significant early result: a victory at the Macau Grand Prix in 1994, one of the most prestigious non-championship Formula 3 events on the calendar. From 1995 to 1997 he raced touring cars for Nissan in the Super Tourenwagen Cup.
In 1998 Maassen moved to the FIA GT Championship, racing a Porsche 911 GT2 for Roock Racing alongside Bruno Eichmann. In 1999 he competed in the Porsche Supercup and drove for Alex Job Racing at Petit Le Mans, winning the GT class alongside Cort Wagner and Dirk Müller.
In 2000, Maassen joined Porsche as a factory driver, with his primary assignment being the American Le Mans Series in a Porsche GT3-R. He partnered with Bob Wollek at Dick Barbour Racing, and the pair won five races including a second consecutive Petit Le Mans GT class victory for Maassen. He also finished second in the GT class at the 24 Hours of Le Mans that year, driving for Skea Racing International.
In 2001, paired with Lucas Luhr at Alex Job Racing, Maassen won at Texas and at the 12 Hours of Sebring in the GT class, the pair breaking through in a season otherwise dominated by the BMW M3 GTRs. He also raced prototypes at the 24 Hours of Daytona for Champion Racing and appeared at Le Mans for Dick Barbour Racing.
The partnership between Maassen and Luhr produced back-to-back ALMS GT championships in 2002 and 2003. In 2002 they won seven of ten GT class races, including a second Sebring victory and a third Petit Le Mans class win for Maassen. In 2003 they won the championship again and claimed Sebring for the third consecutive year. Maassen also recorded his 20th ALMS class win at Miami that season, and took victory at the Grand-Am round at Circuit Mont-Tremblant in a Brumos Racing Fabcar-Porsche.
In 2004, Maassen operated mainly as a third driver for longer endurance races, winning at Sebring and Petit Le Mans for Alex Job Racing and claiming the GT class at the 24 Hours of Le Mans with Petersen Motorsports. In 2005 he focused on development work for the new Porsche RS Spyder, including racing in the car's debut at Laguna Seca.
In 2006, Maassen returned to championship-winning form alongside Lucas Luhr, this time in the LMP2 class with the Porsche RS Spyder, winning the ALMS LMP2 championship. In the first six races they collected one win at Miller Motorsports Park and a second-place overall at Mid-Ohio. Maassen then partnered with Timo Bernhard for the rest of the season and added two more wins. He continued driving for Penske in the ALMS in 2007 and 2008, and at the 2008 24 Hours of Le Mans he finished second in LMP2 for Team Essex alongside John Nielsen and Casper Elgaard.
Maassen's record of multiple ALMS GT class championships, four Petit Le Mans class victories, and four 12 Hours of Sebring GT victories, combined with Le Mans GT class wins in 2003 and 2004, place him among the most decorated Porsche GT drivers of the American Le Mans Series era. His long partnership with Lucas Luhr formed one of the most dominant GT pairings in that championship's history.