He made his debut in the professional football squad of FC Zwolle in the 2004–05 season. He also played for FC Eindhoven before joining RKC Waalwijk.
In 2007 Janssen was called up by Jong Oranje coach Foppe de Haan to be part of his squad for the 2007 UEFA European Under-21 Football Championship where they qualified for the 2008 Summer Olympics. He scored his first and only goal for Jong Oranje to ensure that they won their final warm-up match against Poland before the competition. At the 2007 UEFA European Under-21 Championship, Janssen came on as a substitute and scored both penalties he took during the series (1–1, 13–12 after 32 penalty kicks) in the semi-finals against England. The Dutch went on to retain their 2006 title by beating Serbia 4–1 in the final.