Gardner was a top ten contender for the Heavyweight Championship of the World and retired with a record of 28 wins, 23 by knockout, and 6 losses, a total of 34 fights.

Gardner began his career as a professional boxer on 12 December 1948, in a novice tournament, winning all of his fights by knockout in the first round.

On 14 November 1950 Jack Gardner defeated Bruce Woodcock for the Heavyweight Championship of Great Britain and the British Empire by TKO after 11 rounds.

However, he lost his European Heavyweight Title in November 1951 by decision after 15 rounds to Hein ten Hoff, a 6' 5 boxer at Berlin, Germany.

Gardner retired to his farm in Market Harborough, England, and died at age 52 on 11 November 1978 from a brain tumor.