Born in Billericay, Essex, Harrison, a goalkeeper, started his career at Charlton Athletic without making any first team appearances. From 1996 until 2003 he played 233 Football League games for Barnet and was Player of the Year for three consecutive seasons. After a £10,000 transfer to Leyton Orient where he played 65 League and Cup games – he moved to Peterborough United for the 2005–06 season, but in July 2006 returned to Barnet as player/coach. He was appointed club captain in July 2007.