Martens began his professional career at
K.V. Mechelen, where he was a member of the first team from 1993. For the 1996/97 season he moved to
Roda JC Kerkrade, with whom he won the 1997
KNVB Cup. From January 1998 to June 2003, he played for
SK Sturm Graz, with whom he was Austrian champion in 1998 and 1999 and Austrian cup winner in 1999. He also took part in the 1998/99, 1999/00 and 2000/01
UEFA Champions League seasons. From 2003 to 2005 he ended his career at TuS FC Arnfels and SC Untersiebenbrunn. He also played futsal for 1. FSC Graz. On 29 February 2012, Martens became technical director of
Sint-Truidense V.V. He left the club again for the 2013/14 season and acted as team manager of
FC Schalke 04 until April 2019. ==Honours==