He started his racing career in 1988 in karting. In that year he went to the International German Karting Championship at the start. In the following two years he raced in the Formula Ford 1600 and 1990 was second overall in the German and the European Championship. In the following years he drove in the German Formula 3, where he in 1992 and 1993 respectively the third and 1994 ranked fourth overall. Also in 1994 he won the prestigious Macau Grand Prix. From 1995 to 1997 Maassen started in the German Super Tourenwagen Cup in a Nissan. In 1996, he drove two races in British Formula 3000 and won one of them. Since 1998 he is Porsche works driver and goes to sports car racing at the start.