Quinnell began his senior career with Llanelli RFC at eighteen years of age and after that joined Richmond for three years before joining Cardiff RFC in 1999 where he played fifty-four games for the club between November 1999 and May 2002. He established himself as the backbone of the pack but moved on to Saracens and Worcester for a short while, before returning to the Arms Park in 2003.

Quinnell won his first international cap as a flank forward against Fiji in 1995 at the age of twenty and now has thirty-two international appearances to his name. He was a member of the Rugby World Cup squad in 1999 and has toured with Wales to Argentina and Japan. He was also a member of the team that recorded the famous 32-31 victory over England at Wembley in 1999 and played in every game of the ten match winning streak.

Brother of Scott and son of Derek, Quinnell turned out for his brother's side in his Scott and Rob Howley's testimonial match at the Millennium Stadium; 'Britain & Ireland v The World'.