Watford In 2000, Forrester joined
Watford from
Northampton Town. As Forrester was only ten years old, Watford paid Northampton £13,000 in compensation.
Aston Villa In summer 2007,
Aston Villa signed Forrester from
Watford, despite reported interest from the likes of
Tottenham Hotspur and
Manchester United for the highly rated teen. A transfer tribunal ruled in December 2007, on the compensation due to Watford as Forrester was only 16-years-old at the time he signed for Villa. An initial fee of £250,000 was set which could have potentially risen to £1.2 million as well as a percentage of the sell-on fee. After joining, he immediately helped
Villa's under-18 team to
FA Premier Academy League glory in the
2007–08 season. He also started for Villa's reserve team who in the
2009–10 season, won the Reserve League South title for a third consecutive time. However, he was an unused sub as Villa went on to lose the Play-off Final against Manchester United, champions of the Reserve League North. He was released by Villa in May 2011 after his contract expired.
Kilmarnock (loan) In August 2010, Forrester signed on loan with
Scottish Premier League club
Kilmarnock. He made his
Scottish Premier League debut in the 2–1 win against
St Mirren on 11 September. Forrester claimed his first assist in a 2–1 loss to
Inverness Caledonian Thistle on 23 October, crossing to
Rui Miguel who scored with a header. He played a total of eight games in all competitions for Kilmarnock, before returning to Aston Villa in January 2011. Forrester later joined up with the latter on trial, playing with their reserve side
Jong Ajax, and received praise from manager
Frank de Boer. He scored a hat-trick for Jong Ajax in a 5–2 pre-season victory over
VV GOES on 27 July 2011. Forrester then decided to turn down a move to Ajax to join
League One club
Brentford, managed by
Uwe Rösler, on 4 August 2011 on a two-year contract. He made his Bees debut as an injury time substitute in the 2–1 win at
Exeter City on 16 August 2011.
Doncaster Rovers On 30 June 2013, Forrester left Brentford after his contract expired, opting instead to sign for newly promoted
Championship club
Doncaster Rovers. On 4 January 2014, Forrester scored his first goal for Rovers in an FA Cup third round defeat at home to Stevenage.
Rangers On 31 December 2015, Forrester signed for
Rangers on an initial six-month deal. Forrester said that it was a privilege to join 'one of the biggest teams in Europe' and that he wanted to stay longer than his initial six-month deal. Harry also said that he wanted to develop further as a player by learning from the more experienced players at the club, for example
Lee Wallace and
Kenny Miller. Forrester made his debut for Rangers on 10 January 2016 in a 5–1 win over
Cowdenbeath in the
Scottish Cup. On 13 June 2017, Forrester was told to stay away from training and to find a new club by
Pedro Caixinha.
Tractor In July 2018, Forrester's contract with Rangers was cancelled by mutual consent and he joined Iranian club
Tractor, signing a two-year contract. On 30 August 2018, after two appearances for Tractor, Forrester joined
Tabriz rivals
Machine Sazi on loan.
Move to the United States On 18 January 2019, it was announced that Forrester was signed by
USL Championship side
Orange County SC ahead of the 2019 season. During the 2022 season, Forrester was playing for
NISA side
Los Angeles Force. ==Coaching career==