The son of former Manchester United keeper Harry, John Gregg began his career with Barnsley but did not make the first team there. He came to Carrow Road in 1994 but made just one appearance for the Reserves.. He also had a trial with Wigan Athletic. After a spell with Norwegian side Fram from 1994 to 1996, John joined Glenavon in the Northern Irish league alongside former Canary Youth team star Sean Collins . John moved on to Coleraine in October 1998 (12 appearances, 1 goal) before joining Ballymena United in August 1999 on a free transfer. In the summer of 2000, John impressed for New Zealand side Dunedin Technical.

After returning to play for Cliftonville, he joined Institute as manager in July 2008, staying until 2011. He was Head of Youth Development at Coleraine before joining Glentoran as Assistant Manager in January 2023.