Gardiner wanted to transition to track and field in the shorter sprints, but his high school coach said he was too tall, so he became a 400 m runner.

[6] He went to Moores Island All-Age School, where he was a part of the Exterminators Track and Field Club, Coached by Pastor Anthony Williams.

[10] An appearance at the 2014 World Junior Championships in Athletics resulted in a semi-final run in the 200 m and a sixth-place finish in the 4 × 400 m.[11] His first senior medal came at the 2015 IAAF World Relays, held on home turf, where he gave American competitor Jeremy Wariner a close run in the 4 × 400 m relay, helping the Bahamas to the silver medal alongside Ramon Miller, Michael Mathieu and Chris Brown.

He set the time at the Bislett Games, which brought him victory on his debut on the IAAF Diamond League circuit, finishing ahead of Matthew Hudson-Smith and Pavel Maslák.

[13][14] In 2019, he won the 400 m at the 2019 World Athletics Championships, finishing ahead of American favorite Fred Kerley in a national record of 43.48 s, which also made him the sixth fastest man in history after the race.