At the 2006 Beijing World Junior Athletics Championships Clarke won a bronze medal in the 4 × 400 metres relay. In 2007 he finished first in the 400 metres final of the World Youth Championships to win the gold medal. On 24 July 2009 he won two gold medals (400 m and 4 × 400 m) at the European Junior Championships in a new personal best of 45.59 seconds. 2009 also saw Chris selected for his first Senior Great Britain national team for the World athletics championships in Berlin.