[4] He joined Luton Town's Youth Training Scheme at the age of sixteen but was suspended four months later for stealing a bank card from the son of a couple with whom he was lodging.

[5] Hartson turned professional in 1992 and made his Luton Town first team debut as a substitute in a 1–0 League Cup defeat to Cambridge United on 24 August 1993.

He was strike-partner to Ian Wright, being favoured ahead of Kevin Campbell to fill the gap left by the injured Alan Smith, who retired at the end of the season.

[10][11][8] Following the signing of Dennis Bergkamp, who was preferred up front to partner Wright, Hartson went on to feature under Graham's successors Bruce Rioch and Arsène Wenger.

[19] His final game on 13 January 1999, saw West Ham lose 1–0 in an FA Cup tie against his hometown club, Swansea City.

[22] His time with the Dons was marred by injury, though he still managed a respectable goal tally, first in the Premier League, then the First Division following the club's relegation in 2000.

[32] In December 2002, Hartson scored as Celtic were beaten 2–1 in the away leg of their third round UEFA Cup tie against Celta Vigo.

[35] In the return leg at Anfield, Hartson fired the club's goal of the season with a strike past Jerzy Dudek into the top corner from over 25 yards out.

[36] After scoring the winning goal against Rangers at Ibrox in late April Hartson was plagued by a back injury which meant he missed the rest of the season, including the 2003 UEFA Cup Final.

[39] In March 2004, he dropped out for the remainder of the football season for surgery to his back; however, he recovered in time for Wales' unsuccessful qualifying campaign for the 2006 World Cup.

[48][49] Hartson was sent off on a number of occasions, one being a notable dismissal towards the end of an Old Firm derby, when he was sent off for violent conduct along with Celtic teammate Johan Mjällby and Rangers player Fernando Ricksen.

"[52] On 26 June 2006, Hartson signed a two-year contract with English Football League Championship side West Bromwich Albion in a £500,000 move.

On 5 September 2007, the Western Mail reported that Hartson had rejected a firm approach from Forest and was on the verge of signing for his hometown club Swansea City on loan.

[56] Although Norwich had the option to extend the loan until 31 December, newly appointed manager Glenn Roeder decided to send Hartson back to the Midlands after the initial month period.

[59] On 7 February 2008, Hartson announced his retirement from football, citing his long-term struggles with weight and fitness as major reasons for finishing his career.

[61] He also holds a record for the Wales U21 team by being one of only four people to have scored a hat-trick at that level alongside Craig Davies, Lee Jones and Ched Evans.

"[64] Hartson joined Setanta Sports as a pundit for the 2008–09 season, as part of the broadcaster's coverage of the Scottish Premier League.

[68] John also appeared in the Welsh language travel interest program Am Dro (Going for a Walk)[1] broadcast on 1 January 2024 In July 2009 Hartson received chemotherapy after being diagnosed with testicular cancer which had spread to his brain.

[73] Fowler along with other celebrities including Paul Merson, Matt Le Tissier and John Hartson have all been investigated and issued with a warning by the ASA.

The investigation also concluded that the posts made by these celebrities were making unlawful medical claims about the use of CBD, which is prohibited by the Medicines and Healthcare products Regulatory Agency.