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.
A quick and tricky right-footed left winger, he was highly
regarded by Alex Ferguson and at one point was expected to become a regular player at Old Trafford. Although he did make nine league appearances for United, serious injury and increasing competition for places prevented him from making the expected impact at first-team level. The most notable injury came in April 1994, when he suffered knee ligament damage in a reserve game against Blackburn Rovers due to a reckless tackle by opposing defender Nicky Marker; the injury kept him out of action for over a year.
This came days before the FA Cup semi-final against Oldham Athletic, for which Alex Ferguson was reportedly considering selecting Thornley (at least for a place on the bench) in case Ryan Giggs was not fully fit. Thornley was not fit again until the start of the 1995-96 season. He was rated by contemporaries including David Beckham (one of the players who went on to eclipse him in the first team), as well as club legend and former youth coach Nobby Stiles, as one of the finest young players at the club during that era.
In 1997, Thornley sued Marker and Blackburn Rovers for millions over the tackle, claiming a loss of earnings that resulted from him being kept out of the Manchester United first team because of the injury.
In 1996, he won three caps for England at U-21 level.