[9] Gemili is a member of Blackheath and Bromley Harriers Athletic Club, and also a former football player for Dagenham & Redbridge and Thurrock F.C.

Gemili competed at the 2012 London Olympics where he, after a poor start, came third in the semi-final in a time of 10.06 s, 0.04 s short of qualifying for the final.

He thus qualified for his first major final where he finished fifth with a time of 20.08 s.[19] Gemili won his first senior medal at the 2014 Commonwealth Games coming second to Jamaica's Kemar Bailey-Cole in the 100 m final in a time of 10.10 s.[20] Two weeks later, Gemili took his first senior title, winning the 200 m at the 2014 European Athletics Championships, in an equal personal best of 19.98 s (−1.6 m/s).

[21] Two months later, on 7 July at the Diamond League meeting in Birmingham, he became the 100th man in history to break the 10-second barrier legally, again clocking 9.97 seconds (+2.0 m/s); however, he fell as he crossed the line, picking up a hamstring injury which would cause him to miss the 2015 World Championships.

[24] He was overlooked for individual selection for the 2017 World Championships, following early season injury, but was included in the 4 x 100 m relay team.

Racing both heats and final, he won a historic gold medal as a member of the British quartet in a national and European record, lifting the team to third on the all-time list behind USA and Jamaica.

[26] He narrowly missed out on a medal in the 200 m final, leading the race coming off the bend but ultimately finishing in fourth place.

[27] He ran the first leg of the men's 4 × 100 m relay, winning a silver medal behind the United States in a new European record time of 37.36 seconds.

[28] Off track, in November 2019 Gemili led a group of 20 British athletes including Mo Farah, Katarina Johnson-Thompson and Laura Muir who threatened to take legal action against the British Olympic Association regarding the interpretation of the International Olympic Committee's Rule 40 contained in guidelines which had been recently issued by the association.