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 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. 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 match 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 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, 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.
Huddersfield Town
After loan spells at Stockport County and Huddersfield Town, Thornley finally left United on a free transfer in May 1998 and went on to make 127 appearances for Huddersfield Town, scoring 8 goals and helping them come close to a First Division play-off place – and the chance of a place in the FA Premier League. From June 1999 until November 2000, he was playing under former Manchester United teammate Steve Bruce, who was Huddersfield's manager. When Bruce was dismissed, his successor was another former Manchester United player – Lou Macari, who had left United before Thornley joined them. Thornley's spell at Huddersfield brought mixed fortunes. In his first season they finished 10th in the First Division and in his second they finished eighth, but in his final campaign they suffered relegation to the Second Division.
Aberdeen
In August 2001 he signed a two-year deal at Aberdeen, which gave him another crack at top division football – this time north of the border in the Scottish Premier League. In 30 appearances for Aberdeen, he scored three goals – the opening goal in a 4–2 victory against Motherwell, and both goals in a 2–0 victory over Kilmarnock. He also scored in the Scottish Cup against St Johnstone, and in the Scottish League Cup against Queen of the South.
Blackpool
He returned to England and signed for Second Division club Blackpool on an 18-month contract in November 2002, but was released from his contract at the end of the 2002–03 season.
Bury
Thornley signed for Bury in September 2003, but was released less than two months later after making just five appearances.
Ben then joined Halifax Town on non-contract terms in March 2004, ending his professional career just before his 29th birthday to play in the Conference.
Alongside his younger brother, Rod, a striker, Thornley played for Witton Albion in the Northern Premier League between 2008 and 2010. Rod is also employed by Manchester United as a sports therapist.
Post-playing career
Thornley now works as a television commentator for MUTV.
Personal life
His younger brother, Rod Thornley, also a former academy player at Manchester United, used to be a sports therapist at the club.
