Thomassen started his youth career with
Sarpsborg FK before joining
Sparta Sarpsborg in 2007. In January 2008, he joined
Fredrikstad and played his first game for the club against
Kongsvinger. In May 2009, he signed a contract extension for Fredrikstad until 2012. In November 2011, he was rumored to join the
Seattle Sounders FC of
Major League Soccer. Before the 2012 season, he signed a contract with
Vålerenga. After two seasons with
Vålerenga he signed for
Sarpsborg 08 FF ahead of the 2014 season. ==Career statistics==