Swindon Town 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. Over the next few seasons he made his name as a versatile midfielder capable of playing on the left or in the centre, or at left-back. He also scored goals regularly, with 16 in the 1995–96 season. He made 200 appearances in all competitions, scoring 26 goals, in five seasons at Swindon before joining
Manchester City in January 1997.
Manchester City Horlock was
Frank Clark's first signing for
Manchester City for a transfer fee reported as being between £1.25 million and £1.5 million. 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. The following season, he was a regular in the first team until suffering an injury which sidelined him for four months. During Horlock's absence, Clark was sacked and chairman
Francis Lee later observed that Horlock was one of only two good purchases Clark made. City were struggling at the foot of the table, and were relegated on the final day of the season despite a 5–2 win at
Stoke City, in which Horlock scored the fifth goal. Horlock became Manchester City's primary
penalty taker in the 1998–99 season, a role in which he continued for the next three seasons. 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". Horlock's best season was probably the 2001–02 season where he was deployed in the holding midfield role and helped
Manchester City earn promotion in stylish manner, the team scoring 108 goals. 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. Horlock wore the number 6 shirt Horlock made 232 league and cup appearances, scoring 44 goals, in seven years at Manchester City.
Later career Horlock was signed by
Glenn Roeder for
West Ham United for a fee of £300,000 in August 2003. He made 27 league appearances for West Ham in the 2003–04 season and scored once against Wigan{{cite news |url=https://news.bbc.co.uk/sport2/hi/football/eng_div_1/3284801.stm Horlock signed for
Eastern Counties League Premier Division side
Needham Market as player/assistant manager on a twelve-month contract on 8 August 2008.{{cite news ==International career==