He is a member of the Coventry Godiva Harriers athletics club and is coached by Tony Lester. Early in his career he was successful at both 100 and 200-metre distances, winning English Schools and European Junior titles at both, but in recent years he has concentrated mostly on the longer distance. He has also been a regular member of both the British and, at the Commonwealth Games, English 4 x 100-metre sprint relay teams, to some considerable success. He is a current Commonwealth Games record holder in the relay event.
At the British Championships (and team trials for the 2006 European Championships) in July 2006, Devonish became the first man since Linford Christie in 1988 to win both the 100 m and 200 m races at the event. At the championship finals, he took the bronze medal in the 200 m.
Devonish retained his 100 m title at the British Championships the following year. For the 2007 season Devonish improved his performance in the 100 m with a new personal best and competed in this event at the 2007 World Championships in Osaka rather than the 200 m which he had previously specialised in. Devonish finished 6th in the 100 m final.
Devonish represented Great Britain at the 2008 Summer Olympics in Beijing. He competed at the 4x100 metres relay together with Simeon Williamson, Tyrone Edgar and Craig Pickering. In their qualification heat they were disqualified and eliminated. He also took part in the 200 metres individual, finishing first with a time of 20.49 seconds in his first round heat. With 20.43 seconds in his second round he only placed fourth in his heat, but his time was among the four best losing times and enough to qualify for the semi-finals. There he came to 20.57 seconds and the seventh time in his race, which was not enough for the final.