Speed began his career with Leeds United as a trainee when he left school in June 1988, and was Peter Swan’s boot boy, before he signed a professional contract on 13 June 1988. Leeds manager Howard Wilkinson first noticed Speed in a youth team game while the player was playing in a left back position. Under Wilkinson, he made his first team debut aged 19 in a goalless draw against Oldham Athletic in the Football League Second Division. He would go on to play in nine out of the ten outfield positions, although he was predominantly a left-sided player.

He went on to play a key role, playing 41 of a possible 42 games and scoring seven goals, as Leeds won the Football League First Division championship title in 1992, as part of a midfield that also comprised Gordon Strachan, Gary McAllister and David Batty – former Leeds manager Eddie Gray considered it to be one of the greatest midfield line-ups in the modern era. Wilkinson named Speed as the club’s player of the season.

In September 1992, Speed was pivotal in Leeds’ UEFA Champions League first-round tie against VfB Stuttgart. Stuttgart had won 3–0 at home and looked to be going through already. Speed helped Leeds to a 4–1 victory at Elland Road, scoring one of them with a “superb left-foot volley” which he later described as his best ever goal. Although Leeds went out of the tie on the away goals rule, they were later re-instated as Stuttgart had fielded an ineligible player.

On 4 October 1995, Speed scored the winning goal as Leeds beat second division side Notts County in the second round of the League Cup. The match had seemed to be going to extra time but Speed’s 90th-minute winning goal had stopped Leeds from needing a replay to overcome their lower league opponents.

Speed featured in the 1996 Football League Cup Final defeat by Aston Villa. Before transferring to Everton, Speed played in 312 games for Leeds United, scoring 57 goals.

A childhood Everton fan, Speed was signed by Joe Royle before the 1996–97 season, for a fee of £3.5 million. He made his debut on 17 August 1996, scoring against Newcastle United. In November of that year, Speed scored a hat-trick – the only one of his career – as Everton beat Southampton 7–1 at Goodison Park. Speed finished the season with 11 goals, joint top-scorer with Duncan Ferguson. Speed was also voted Everton Player of Year for his performances during his first season at the club.

Speed married his fiancée Louise in 1996; the couple had two children, Thomas and Edward. He enjoyed quizzes, but he was better at the subjects of geography and history than football.

Howard Kendall succeeded Royle as the manager at the start of the 1997–98 season, and nominated Speed as club captain. However, by the turn of the year the relationship between Speed and Kendall had soured. Speed played his last game for the club on 18 January 1998, scoring in a 3–1 win over Chelsea. He did not travel for their next game, away to West Ham United, and was subsequently sold to Newcastle for £5.5 million. The reasons underlying Speed’s departure were never revealed. He told the Liverpool Echo: “You know why I’m leaving, but I can’t explain myself publicly because it would damage the good name of Everton Football Club and I’m not prepared to do that.”

Speed was signed for Newcastle United on 6 February 1998 by Kenny Dalglish, for a fee of £5.5 million. He made his debut as a starter in a 1–0 home Premier League defeat to West Ham United the next day. His first goal came in his fifth match, the sixth round proper of the 1997–98 FA Cup. At home to Barnsley, he scored the second goal of a 3–1 home win with a 27th-minute strike. His only league goal of the 1997–98 season came in a 3–1 home win over Chelsea in the penultimate league match on 2 May, with Newcastle’s third in the 59th minute. He finished the season by playing the 1998 FA Cup Final, which Newcastle lost 2–0 to Arsenal on 16 May.

Speed featured in the club’s FA Cup Final defeat to Manchester United on 22 May 1999. On 19 September 1999, he scored a goal in Newcastle’s 8–0 Premier League victory over Sheffield Wednesday in which his teammate Alan Shearer scored a joint-record five times. He also played in the UEFA Champions League with Newcastle in the 2002–03 season, scoring the 58th-minute equaliser in a 2–1 home victory over Dynamo Kyiv in the group stage on 29 October 2002.

Speed broke the record held by Dean Saunders for most caps by a Welsh outfield player when he won his 76th against Finland in a Euro 2004 qualifier on 10 September 2003. He captained the side to a 1–1 draw. Speed retired from international football in 2004 after the side’s 3–2 defeat to Poland in a 2006 World Cup qualifier.

Speed moved to Bolton Wanderers on a two-year deal in a £750,000 switch from Newcastle United in July 2004. He became the first player to make 500 Premier League appearances when he played in Bolton’s 4–0 victory over West Ham United in December 2006.

On 1 May 2007, Speed was named as the first-team coach for Bolton after Sam Allardyce stepped down from his job as manager. However, in October he left the coaching job and returned to being just a player with the club. Conflicting reports of the incident claimed then manager Sammy Lee relieved him of his duties to concentrate on playing but Speed, in an interview with the Bolton News, claimed he chose to step down.

Speed scored a header for Bolton against Reading on 25 August 2007, making him, at the time, the only player to have scored in every Premiership season to date. Ryan Giggs later matched this achievement on 20 October 2007, and passed it on 8 February 2009. It had been reported in December 2007 that Speed was a target for Derby County, but it was confirmed on 24 December 2007 that he would join Sheffield United on loan on 1 January 2008, with a view to a permanent move for a fee of about £250,000.

Speed made his debut on the day of his transfer – being named in the starting eleven for a 0–0 away draw against Wolverhampton Wanderers on New Year’s Day 2008. He played regularly for the rest of the season and deputised as captain when Chris Morgan was unavailable. Having seen his penalty parried, Speed netted the rebound to score his first goal for United in a victory over Coventry City in March of that year. He went on to finish the season on three goals for United, scoring both goals in a 2–1 home victory over Bristol City in the penultimate game of the 2007–08 season. In an interview with BBC Wales on 10 May 2008, Speed hinted that the 2008–09 season may be his last playing before moving into coaching or management but stated that he had not yet made up his mind.

Speed started the next season as a regular fixture in the United midfield but succumbed to a back injury in November 2008. Despite undergoing surgery to correct the problem he failed to regain his fitness and missed the rest of the season, concentrating instead on a coaching role at the club. In June 2009, he was approached by Swansea City with regards to taking on the managerial role left vacant by Roberto Martínez. He continued training and completed the 2010 London Marathon in aid of the Sir Bobby Robson Foundation, the cancer charity founded by Speed’s former Newcastle manager, who had died of the illness in 2009. Speed’s finishing time was 3 hours, 49 minutes and 22 seconds.

With the 2010–11 season only three games old, Sheffield United manager Kevin Blackwell was dismissed with Speed being confirmed as the club’s new manager on a three-year contract on 17 August 2010. On 21 October 2010, Speed was given a one-game touchline ban and a £22,000 fine by The Football Association for his unsporting behaviour at the home fixture against Watford on 2 October 2010. On 11 December 2010, Speed was linked with the vacant Wales manager job along with John Hartson, Brian Flynn, Chris Coleman and Lars Lagerbäck. Sheffield United confirmed that they had given Speed permission to talk to the Football Association of Wales over the vacant position. With just four months managerial experience, Speed was confirmed as the new Welsh national team manager on 14 December 2010 succeeding John Toshack who had stepped down in September 2010. Sheffield United released Speed from his contract after compensation was paid by the FAW. Former Welsh national team manager Mark Hughes supported the move to appoint Speed, saying, “He’s got a strong personality, he’s good with people, (the players) will relate better to Gary than they perhaps did to the previous manager.”

Speed was appointed a Member of the Order of the British Empire (MBE) in the 2010 Birthday Honours for services to football.

Speed’s first game as Wales manager was 8 February 2011 in the inaugural Nations Cup, which the Republic of Ireland won 3–0. Speed’s first competitive match was the Euro 2012 qualifier at home to England 26 March 2011 and Speed appointed twenty-year-old Aaron Ramsey captain, making Ramsey the youngest ever Wales captain. Wales lost to England 2–0, and in August 2011 Wales attained their lowest ever FIFA ranking of 117th. This was followed by a 2–1 home win against Montenegro, a 1–0 away loss to England, a 2–0 home win against Switzerland and a 1–0 away win against Bulgaria. Consequently, in October 2011, Wales were ranked 45th in the world by FIFA. Speed’s last game as manager of Wales was on 12 November 2011, a 4–1 home win in a friendly match against Norway. On 21 December 2011, the day of the final FIFA rankings of the year, Wales were awarded the title of ‘Best Movers’ of the year having gained more ranking points than any other nation in 2011.

On 26 November 2011, Speed appeared as a guest on the BBC One television programme Football Focus, with presenter Dan Walker later describing Speed as being in “fine form”. After the programme finished at 1 pm, Speed chatted to various other pundits at the MediaCity studios in Salford before joining former Newcastle United teammate Alan Shearer to watch their old club play against Manchester United at Old Trafford, a short walk from the studios across the Manchester Ship Canal. After the match ended, at 5 pm, Speed drove home to Huntington, Cheshire, about an hour’s drive from Old Trafford.

The following morning, just before 7 am, his wife Louise found his body hanged in the garage of his home. She telephoned the ambulance service at 7:08 a.m. and the police were also informed. They confirmed that he was dead and they announced that they were not treating his death as suspicious. Although the facts were not fully established, it was reported that Speed’s death had been a suicide. His death was announced to the public by the Football Association of Wales a few hours later. An inquest into Speed’s death convened on 29 November 2011. The inquest was adjourned until 30 January 2012.

Speed’s funeral, attended by around 250 members of his family and close friends took place in the village of Hawarden on 9 December 2011. In keeping with his wife Louise’s wishes, only small crowds gathered on the main road outside St Deiniol’s Church and no members of the press were admitted. Speed was later cremated in Pentre Bychan Crematorium, Wrexham.

In March 2012, Speed’s eldest son Ed was selected for the Wales Under-16 squad.

On 25 September 2012, Everton visited Leeds United in the League Cup. Starting from the 11th minute of the fixture, both sets of supporters chanted Speed’s name for 11 minutes.

Speed played for Wales at youth level and earned three caps for the under-21 team. He played 85 times for Wales, making him the second highest capped player only behind goalkeeper Neville Southall who made 92 appearances until Speed was surpassed by Chris Gunter in 2018. Speed captained Wales on 44 occasions. His first appearance for the national side had come on 20 May 1990 in a 1–0 friendly win against Costa Rica at Ninian Park as a second-half replacement for Glyn Hodges. He did not score until his 27th cap, a 3–2 away defeat in Moldova in a Euro 96 qualifier on 12 October 1994. Speed’s first appearance as captain was during his 46th cap, a 3–0 away win in Malta on 3 June 1998.

As a boy, Speed was coached by Barry Bennell, who was later convicted as a serial child sex offender. In February 2018, after Bennell’s conviction, an anonymous victim of the coach told Al Jazeera that he had witnessed Speed being abused. However, police twice interviewed Speed during earlier investigations into Bennell’s behaviour, and he said that he was never harmed by him; the inquest into Speed’s death found no links to Bennell.