Horlock started his career as a trainee with West Ham United in 1989 but did not graduate to the first team and moved to Swindon Town in August 1992 where he spent a single season in the Premier League in 1993–94.
[3] He made 200 appearances in all competitions, scoring 26 goals, in five seasons at Swindon before joining Manchester City in January 1997.
[5] He made his debut in a 4–1 Division One win against Oxford United and scored his first goal for Manchester City three weeks later against his former club Swindon.
Later that season Horlock was sent off in a match against AFC Bournemouth in unusual circumstances, receiving a second yellow card for "walking towards the referee in an aggressive manner while asking a question".
[9] One of the most notable moments of his career was the 1999 season play-off final against Gillingham when he scored the first goal of the comeback when Manchester City won on penalties after being 2–0 down until the last five minutes of the game.
[22] Horlock signed for Eastern Counties League Premier Division side Needham Market as player/assistant manager on a twelve-month contract on 8 August 2008.
[23] Horlock was first capped in April 1995 versus Latvia and went on to make 32 appearances for Northern Ireland,[2] before retiring from international football in 2003 for personal reasons.
He left in 2015 when he was appointed as manager of Isthmian League Division One South side Chatham Town on 15 October 2015.
[26] Horlock combined both his Needham Market and Chatham Town roles with the assistant manager's job to the Northern Ireland under-21 team, a position he was appointed to in August 2013.