McAvoy was born on 21 April 1979 in Glasgow, Scotland, to James, a bus driver, and Elizabeth (née Johnstone), a nurse. He was raised on a housing estate in Drumchapel, Glasgow by his maternal grandparents (James, a butcher, and Mary), after his parents divorced when James was 11. He went to St Thomas Aquinas Secondary in Jordanhill, Glasgow, where he did well enough and started 'a little school band with a couple of mates'.
