Gardener was born in Bath, Somerset to a Jamaican father and British mother, and started his career at the World Junior Championships in 1994, where he was placed second in the individual 100 m and bettered this to take his first gold medal as part of the 4 × 100 m relay team.

2000 saw Gardener go one better in the European indoor 60 m taking gold as well as breaking the national 50 m record with a time of 5.61 s. Unfortunately he became injured during the summer and although making it to Sydney for the Olympics he did not progress through the early rounds.

Gardener made the squad for the 2004 Summer Olympics where he competed in the 100 m and won the gold medal in the 4 × 100 m relay with Darren Campbell, Marlon Devonish and Mark Lewis-Francis in a 38.07 s, their season's best and a narrow upset win over the United States.

[1] In March 2007, Gardener won his fourth European Indoor 60 m gold after fears that he may have to miss defending his title as his wife, Nancy, was due to give birth.

In 2008 Jason took part in a project to qualify four athletes from diverse sports in the British 2 Man Bobsleigh Championships, after just two weeks of training.