He is a fly half. He was one of the quickest players in history to reach 500 Premiership points; his goal-kicking and place kicking being his particular strengths. Whilst at Gloucester he started in the 2003 Powergen Cup Final in which Gloucester defeated Northampton Saints. In the final Mercier scored four conversions, three penalties and a drop goal.
In June 2010 he joined Aironi for their first season in the Magners League.