Kelly featured for clubs Arsenal, Queens Park Rangers, Leicester City, Notts County, AFC Bournemouth, Kettering Town and Torquay United.

He made 16 appearances in the 1969–70 season, including scoring a goal in the Gunners' 4–3 aggregate win over Anderlecht in the Inter-Cities Fairs Cup final.

Kelly made 34 appearances as Arsenal won the Double; he came on as a substitute in the FA Cup final against Liverpool, and scored the Gunners' equaliser in extra time in a goalmouth scramble.

George Graham made a claim for the goal, but the replays show Kelly was the last Arsenal player to touch the ball.

After making a transfer request, he joined Queens Park Rangers in September 1976, having played 222 times and scoring nine goals altogether for Arsenal.