A former youth player at Blackburn Rovers, the midfielder, who is known for his crossing ability, won a move to Preston North End in 1992 after impressing at non-league Northwich Victoria. He moved on to Cambridge United, who in turn loaned him back to Northwich Victoria. He returned to Preston North End in 1993, and established himself in the first team during his second spell at the club. Sold on to Lincoln City in October 1995 for a £25,000 fee, his performances at Lincoln earned him a place on the PFA Third Division Team of the Year in 1996–97, and also won him a £500,000 move to Port Vale in September 1997. After being named as the Port Vale F.C. Player of the Year in 1998, he was sold on to Premier League Wimbledon for £2 million in October 1998. He was dogged by injuries at the club, and following loan spells with Preston North End and Walsall, he moved on to Cardiff City for a nominal fee in March 2003. He signed with Queens Park Rangers in June 2003, after went on to spend seven years with the club. During his time at QPR he helped the club to win promotion out of the Second Division in 2003–04, and twice served as caretaker-manager.
He joined Wycombe Wanderers in February 2010, following a short loan period. He was named in the PFA League Two Team of the Year in 2010–11, as Wycombe won promotion out of the division. He was appointed as the club's manager in November 2012, after a short period as caretaker-manager. He retired from regular playing appearances to concentrate on his management duties in April 2013, but remains registered as a player.
Ainsworth started his career as a trainee with Second Division club Blackburn Rovers, but despite forming a useful partnership with Peter Thorne he was not offered professional terms at the end of his two-year training period. He dropped into the Football Conference with Northwich Victoria, scoring his first goal in the 3–1 home victory over Cheltenham Town on 26 October 1991. His form attracted the attention of Third Division club Preston North End and, in January 1992, they offered him a second chance at the professional game. Released at the end of the 1991–92 season, Ainsworth joined First Division Cambridge United under the management of John Beck. Ainsworth had a spell on loan with former club Northwich Victoria, making his second debut for the "Vics" in a 2–1 home victory over Bromsgrove Rovers on 5 September 1992. Beck was sacked by Cambridge in October 1992 but was swiftly appointed manager at Preston North End, and in December returned to the Abbey Ground to sign Ainsworth.
Ainsworth's second spell with Preston lasted three years. Preston reached the Third Division play-off final in 1994, and Ainsworth played in the Wembley final, but could not prevent Wycombe Wanderers winning the game 4–2. Preston reached the play-offs again the following year, but exited at the semi-final stage with a 2–0 aggregate defeat to Bury.
Beck left Preston following the disappointments at the play-offs, and was appointed manager at Third Division rivals Lincoln City.
In October 1995 he signed Ainsworth for a third time, this time for a fee of £25,000. In 1996–97, Ainsworth scored 22 goals to make him the division's second-highest goalscorer after Wigan Athletic's Graeme Jones. For this achievement he was named on the PFA's Third Division Team of the Year. A popular player at Sincil Bank, Ainsworth was twice Player of the Season and was voted fourth in the club's top 100 legends after a poll conducted in May 2007.
In September 1997 he signed for John Rudge's First Division Port Vale for a club record £500,000 fee; he was signed to replace Jon McCarthy and Steve Guppy, wingers who had been sold for a combined £2.35 million earlier in the year. He became the club's Player of the Year for his performances in the 1997–98 season, helping the "Valiants" to narrowly avoid relegation above Manchester City and local rivals Stoke City. He was though criticized for an 'ugly' tackle on Sheffield United's Dane Whitehouse in November that resulted in the player's early retirement due to a serious leg injury sustained from the challenge.
He moved to Premier League side Wimbledon in November 1998 for a £2 million fee, again a club record for Port Vale. His time at Wimbledon would be dominated by struggles again injury. He played just eight times in the 1998–99 season, failing to score. He was limited to just two appearances in 1999–2000, though in the first of these, a 3–3 draw with Newcastle United at St James' Park, he scored two goals, including a last minute equaliser. At the end of the season the "Dons"' were relegated into the First Division.
He regained his fitness by January 2001, and managed twelve league and six FA Cup games in 2000–01. After missing the entire first half of the 2001–02 campaign, he returned to fitness only to find that he had lost his first team place due to the form of promising youngster Jobi McAnuff. Ainsworth was told that he was to be released in summer 2002, and was allowed to join former club Preston North End on loan in April 2002. He played five games for Preston, scoring one goal against Coventry City. He returned to Wimbledon to find that he was to be offered a new contract, after the club announced it was considering a potentially lucrative move to Milton Keynes.
After six appearances for the "Dons" in 2002–03, he was allowed to join Walsall on loan in December. He scored for the "Saddlers" against Nottingham Forest, but his loan deal was not extended. He scored twice in nine games upon his return to Wimbledon, before March 2003, when he moved to Second Division club Cardiff City on a short-term deal for a "small fee". The "Bluebirds" won promotion to the First Division as play-off winners, though Ainsworth played no part in the victory over Queens Park Rangers in the final at the Millennium Stadium. His stay with the club was brief, as manager Lennie Lawrence told him that he could not guarantee him a first team spot. Both before and after his release from Cardiff he was linked with a permanent move back to Walsall, but manager Colin Lee had limited funds available for new players.
Ainsworth signed for Second Division club Queens Park Rangers in July 2003. He scored twice on his debut, in a 5–0 win over Blackpool on 9 August 2003. Sixteen days later he made it five goals in four games with a brace against Rushden & Diamonds, one a volley from 35 yards, and another a powerful drive from the same distance. QPR won promotion to the First Division as Second Division runners-up at the end of the 2003–04 season.
Despite suffering with a knee injury in 2004–05 that limited him to 23 appearances, manager Ian Holloway offered him a new contract at the end of the season after telling the media that "he's an important player to have around". During the disappointing campaign of 2005–06, Ainsworth came second in the Player of the Year awards behind Danny Shittu, and finished joint-top-scorer on eleven goals with Marc Nygaard.
During the 2006–07 season, Ainsworth struggled with injuries, and his season ended in April after a scan revealed that he had broken his leg in a win over Luton Town. During the 2007–08 season, he assisted new manager Luigi De Canio in addition to his 25 games, and stated his intention to move into coaching once his player career is over.
In May 2008, Ainsworth accepted a player coach role under new QPR boss Iain Dowie. Dowie was sacked in October 2008, and Ainsworth was appointed as caretaker-manager. On taking up the reins, Ainsworth announced that "QPR is very strong with the result of what happened and will stay strong, nothing is going to break us". During his time in charge QPR managed to test Manchester United at Old Trafford in a League Cup encounter that was only settled by a 76th minute Carlos Tevez goal. This performance caused Ainsworth to believe that the club could achieve promotion to the Premier League. He remained on the club's coaching staff after Paulo Sousa was appointed as manager in November. Sousa's reign did not last long however, and on 9 April 2009, Ainsworth again took the role of caretaker-manager.
He was considered for the vacant management position at former club Lincoln City in September 2009.
On 20 November 2009 Ainsworth joined League One club Wycombe Wanderers on a monthlong loan. He went on to sign an eighteen-month contract with Wycombe in February 2010. He scored his first goal for the club in a 2–2 draw with Exeter City on 20 March 2010. However the club were relegated at the end of the season.
The club achieved promotion out of League Two at the first attempt in 2010–11, after finishing in the third automatic promotion spot, a single point ahead of Shrewsbury Town. Ainsworth scored eleven goals in 46 appearances, and captained Wycombe to the 3–1 victory over Southend United that ensured the club promotion on 7 May 2011. Later that month Ainsworth signed a new one-year contract with the club. For his performances he was named on the PFA League Two Team of the Year. However the "Chairboys" were relegated straight back down in 2011–12, with Ainsworth scoring twice in 32 games.
He retired from professional football on 27 April 2013, after playing his final game against his former club, Port Vale at Adams Park. Although he agreed to sign a two-year contract to remain as Wycombe manager. On 30 August 2016, Ainsworth came on as a substitute in the EFL Trophy against Northampton Town, and provided an assist for Garry Thompson in a 3–0 victory.
Following the sacking of Gary Waddock, Ainsworth was appointed as Wycombe Wanderers's caretaker-manager on 24 September 2012. Before he took charge, Wycombe had picked up just four points from their seven league games.
After a loss and a draw, he led the team to victory over Torquay United on 6 October, and provided the cross himself for the opening goal of the game. In October, he signed Portuguese winger Bruno Andrade on loan from QPR. A further win over Fleetwood Town, Andrade scoring the only goal of the game, lifted the club out of the relegation zone. Ainsworth was appointed as the club's manager on a permanent basis on 8 November. In December 2012, the "Chairboys" gained ten out of a possible 15 points to rise ten points clear of the relegation zone – Ainsworth was recognized for this achievement by being nominated for the division's manager of the month award. In April 2013, Ainsworth signed a new two-year managerial contract, and also announced his retirement as a player.
Ainsworth lost striker Joel Grant to Championship club Yeovil Town at the start of the 2013–14 season, and signed attackers Jon-Paul Pittman, Steven Craig, and Paris Cowan-Hall to replace him. Defenders Dave Winfield and Charles Dunne also left Wycombe for clubs in higher divisions (though Dunne was loaned back to Wycombe for the season as part of the transfer). He signed right-back Nick Arnold and midfielder Billy Knott on loan. In the January transfer window he sold 18-year-old centre-back Kortney Hause to Wolverhampton Wanderers, and signed 19-year-old former Arsenal winger Anthony Jeffrey on a free transfer. On the final day of the season Wycombe needed to win at Torquay United and hope results went their way elsewhere in order to stay up; they won their match 3–0 and finished outside of the relegation zone on goal difference.
He completely restructured the defence with free signings in preparation for the 2014–15 season, bringing in Joe Jacobson, Peter Murphy, Sido Jombati, and Aaron Pierre, whilst also bringing in striker Paul Hayes from Scunthorpe United. Hayes finished as top-scorer as Wycombe went on to end the campaign in fourth place, one point behind automatically promoted Bury. They reached the play-off final at Wembley Stadium, where they lost out on penalties to Southend United following a 1–1 draw. Ainsworth signed a new five-and-a-half year contract in January 2015 and was named as that season's LMA Manager of the Year for League Two.
Wycombe finished the 2015–16 season in 13th place, and Ainsworth admitted that the club was in a precarious financial position despite taking Premier League club Aston Villa to a replay in the Third Round of the FA Cup. Ainsworth was named as the EFL's manager of the week on 3 January 2017, after his side recorded two consecutive victories to begin the year fifth in the table. Wanderers ended the 2016–17 season in ninth place, one point and two places outside of the play-offs. They also enjoyed more cup runs, exiting the FA Cup at the fourth round with a 4–3 defeat to Tottenham Hotspur at White Hart Lane and losing 2–1 at Coventry City in the semi-finals of the EFL Trophy.
Ainsworth is able to play on the right-wing or as an attacking midfielder, and has good crossing ability; he was also known as a pacey player in his 20s and early 30s.
Ainsworth is a very attack-minded manager, and likes his teams to play aggressively and with a high press.
During his playing days Ainsworth picked up the nickname 'Wild Thing' due to his appearance and his rock star ambitions; he was in a band called 'APA' with Wimbledon teammates Chris Perry and Trond Andersen; he was also in a band called 'Dog Chewed the Handle'. He is considered a cult hero at Preston, Wimbledon, QPR, Port Vale, Lincoln City, and Wycombe Wanderers.
On 21 October 2010, he represented The Football League at the unveiling of the Footballers' Battalions memorial on the site of the Battle of the Somme.As of match played 16 September 2017
As a playerPFA Football League Third Division Team of the Year: 1996–97
Port Vale F.C. Player of the Year: 1997–98
PFA Football League Two Team of the Year: 2010–11
As a managerLMA Manager of the Year (Football League Two): 2014–15