Horsfield announced his retirement from football in 2008 after being diagnosed with testicular cancer, but after successful treatment, he resumed his career, signing a six-month contract for Lincoln City in January 2009.

In July of that year, he was appointed player-assistant manager at Port Vale under Micky Adams, and the following summer, he took up coaching full-time before leaving the game completely in May 2012.

[9][10] However, he also sustained a potentially career-threatening knee injury;[4] after making a full recovery he returned to Halifax for a fee reported as £4,000.

[7] On the last day of the 1996–97 season, needing to beat Stevenage Borough to avoid relegation from the Conference, Horsfield scored the goal that clinched a 4–2 victory.

[15] Together with teammate Mark Bradshaw, Horsfield was selected for England's semi-professional representative team for a match against their Dutch counterparts, but injury prevented him from playing.

[22] After new manager Jean Tigana made it clear that Horsfield's aggressive style and perceived lack of pace and mobility would not fit the way he wanted his new team to play, and brought in Louis Saha as his main striker, the player agreed to join Fulham's First Division rivals Birmingham City.

[23][24] In July 2000, Horsfield signed a five-year contract with Birmingham, who paid a club record fee of £2.25 million, £350,000 of which went to Halifax Town under the sell-on clause.

[18][25] He was their top scorer in his first season, finding the net on twelve occasions,[26] and his two goals in the second leg of the League Cup semi-final helped the club reach their first major final for nearly 40 years.

[27] In the starting eleven for the final against his boyhood heroes Liverpool, Horsfield had been substituted by the time Birmingham lost in a penalty shoot-out.

[4] Horsfield had promised a donation to his first club, Athersley Rec, if he ever reached the Premiership; a few days after the play-off victory he gave them £25,000 towards improving their facilities.

[32] He also scored in the return fixture at Villa Park, an eventful game in which he ended up keeping goal when Nico Vaesen was injured after Birmingham had used all their substitutes.

[33] Horsfield missed games through suspension and surgery, and was frustrated by manager Steve Bruce preferring to use him as a specialist substitute; after he came on to score a late winner against local rivals West Bromwich Albion, Bruce described him as "a manager's dream", saying that "when you're tiring, the last thing you need is Big Horse rampaging at you".

When Bruce could not guarantee him a regular place in the team, Horsfield moved on to Wigan Athletic, signing a three-year contract in September 2003.

Combined with results from other matches, the 2–0 win ensured Albion's escape from relegation as the first club to survive in the Premier League after being bottom at Christmas.

Horsfield and Warnock wanted to terminate the loan prematurely, but West Bromwich Albion had already agreed to sell the player to United at the end of the season and refused to return on the deal.

[54] He made his debut for the club in their 1–1 draw with Luton Town on 3 February and scored his first goals for them in their 3–0 victory over local rivals Coventry City two weeks later.

[59] Horsfield had a trial at Chesterfield during the summer of 2008 but rejected a move to Saltergate because he wanted to join a club closer to his home in Leicester.

[63] He linked up with Lincoln City, managed by former Halifax Town teammate Peter Jackson, for a week's training to assess his fitness levels,[64] and after an extended trial period, signed a short-term playing contract to run from 2 January 2009 until the end of the season.

[68] Horsfield played regularly during his time with Lincoln, but scored only that one goal, and at the end of the season the club decided not to renew his contract.

[72] The club reached the third round of the League Cup, but after three consecutive defeats, Adams placed the entire squad on the transfer list.

Adams said that Horsfield would have a heavier workload over the 2010–11 season, which would include many hours of scouting to "formulate a catalogue of players [and] get to know all the leagues at all levels", and he would retain his playing registration for emergencies.

[81] Vale were beaten 5–0 by Rotherham United in his first game in charge,[82] but rallied to beat Burton Albion 2–1, before Jim Gannon was appointed manager.

[85] The national media reported that Gannon granted Horsfield's request for a day off for family reasons but then wrote to the board complaining about his conduct.

[91] In December 2011, loan striker Guy Madjo celebrated his first goal for the club by running over to Horsfield on the touchline, "to say thank you for all the finishing (practice) that we have been doing.

[95][96] Horsfield decided to make his illness public to help spread awareness of the disease and lent his support to a Premier League-backed project launched in 2009 to promote men's health issues.