Gardner went to St Columbas Roman Catholic School and St. Bernard's Catholic High School at Primary and Secondary level respectively. He started 6th form but decided to concentrate on his love affair with the gym.
Gardner made his Super League debut in a match against London Broncos on 28 March 2002, which St Helens lost. Having won Super League VI, St Helens contested the 2003 World Club Challenge against 2002 NRL Premiers, the Sydney Roosters. Gardner played at on the wing in Saints' 38-0 loss. He established himself as a first team regular in 2003 and has now appeared more than 220 times for St Helens, scoring more than 118 tries. Gardner played for St Helens on the wing in their 2006 Challenge Cup Final victory against the Huddersfield Giants. Gardner also finished the 2006 season as St. Helens leading try scorer with 29 tries in all competitions.
Gardner was named in the Great Britain training squad throughout 2006. He scored on his international debut in XXXX Test Match against New Zealand on 27 June 2006.
Gardner was selected in June 2007 in a young Great Britain team named by new coach Tony Smith for their match against France at Headingley. On 25 August, he scored two tries for St Helens at Wembley Stadium to help Saints win their 11th Challenge Cup.