 Thornley was born in Bury, Greater Manchester.He is most widely known for his career as a youth and reserve team player at Manchester United, where he played in the left wing position from 1991 to 1998 (becoming a professional in 1992 after helping United win the FA Youth Cup) but was occasionally used as a striker.Thornley made his debut for United on 26 February 1994 as a substitute in the Premier League match against West Ham United which ended in a 2–2 draw at Boleyn Ground.