Early career Cahill began his career with AFC Dronfield, a
Sunday league team, where he played until he was 15. During his time there, he was scouted by a number of clubs, including
Barnsley, Sheffield Wednesday,
Derby County and
Aston Villa, the last of which he signed for as a member of the club's
Academy. "All my friends played for the local side, AFC Dronfield, so I went along and started playing centre half and in centre midfield for them", said Cahill in an interview with the
Manchester Evening News.
Aston Villa On 8 November 2004, Cahill was
loaned to
Burnley. He made his first-team debut the following day in the fourth round of the
League Cup, playing the full 90 minutes as they lost 0–3 to
Tottenham Hotspur at
Turf Moor. On 13 November, he made his debut in
the Football League, in the Clarets' 1–0 home
Championship win over
Nottingham Forest. He made 32 appearances on loan, 27 of which were in the league, scoring once on 3 January 2005, the only goal against
Stoke City at the
Britannia Stadium. At the end of the season, nineteen-year-old Cahill was handed the accolade of Burnley's Young Player of the Year. Cahill made his Villa debut on 20 September 2005 in the second round of the League Cup, playing the entirety of an 8–3 win over
Wycombe Wanderers away at
Adams Park. On the following 1 April, he made his
Premier League debut as a 52nd-minute replacement for
Ulises de la Cruz in the 5–0 defeat at
Arsenal. Fifteen days later, he scored his first goal, an overhead kick against
rivals Birmingham City, to put Villa back into the lead as they went on to win the match 3–1. On 19 September 2007, Cahill joined Championship team
Sheffield United on a three-month loan deal. He made his debut in a 3–2 away defeat at
Crystal Palace three days later, and scored his first goal for the Blades as the only one of a win at Stoke City on 10 November. On 27 November, he scored again in a 0–3 win at
Charlton Athletic. Under the terms of his loan agreement, Cahill returned to Villa Park in mid-December 2007 having made 16 appearances and scoring two goals. Speculation surrounded Cahill's future in the January 2008
transfer window, with Birmingham City,
Bolton Wanderers and Turkish club
Beşiktaş all reportedly showing an interest.
Bolton Wanderers in 2011 Bolton scout
Colin Harvey was impressed by Cahill and on 30 January 2008, Cahill joined them on a three-and-a-half-year deal. Three days later, he made his debut for Bolton in their 0–2 victory over
Reading away at the
Madejski Stadium. Cahill was sent off on 26 August 2008 in the second round of the League Cup, receiving a straight red card in the 38th minute for a foul on
Giles Coke as the team lost 1–2 at home to
Northampton Town. On 5 October, Cahill scored his first Bolton goal in a 1–3 victory at
West Ham United, and he added two more over the campaign in 4–1 wins over
Sunderland and
Middlesbrough. In August 2009, 18 months after joining Bolton, Cahill signed a new three-year contract with the club. He suffered a blood clot in his left arm after a training injury which forced him to pull out of the home match against
Fulham on 6 February 2010; he spent the weekend in hospital. The treatment for the ailment meant that Cahill could not play for a number of months. He nonetheless cemented his place at the heart of the Bolton defence in the
2010–11 season, making 36 Premier League appearances and scoring three goals, all of which came at home. On 13 August 2011, Cahill scored on the opening day of the
2011–12 Premier League season against
Queens Park Rangers at
Loftus Road in a 4–0 victory from 25 yards out in the 45th minute. In December 2011, before the
January transfer window, Bolton manager
Owen Coyle admitted that Cahill may be sold as his contract will expire at the end of the
2011–12 season. Coyle said, "If he [Cahill] does move, there is no doubt there will be top clubs clamouring for Gary." This led much speculation that Cahill could move to higher-placed clubs like Arsenal,
Chelsea,
Manchester United or Tottenham Hotspur, all of which showed interest. On 23 December, Coyle confirmed that they had started negotiations with Chelsea about a possible transfer, and a week later a transfer fee was agreed. Despite being on the brink of joining Chelsea, Cahill continued to play for Bolton, scoring the crucial goal in the 1–2 away win against
Everton at
Goodison Park on 4 January 2012. Two days after that, Coyle said that Cahill was not going to play in their FA Cup clash against
Macclesfield Town, as it would leave him
cup-tied for Chelsea.
Chelsea 2011–12 season in 2012 After more than two weeks, the clubs had agreed a fee for him, and on 16 January 2012 Cahill signed for Chelsea for what was believed to be around the region of £7 million, and the negotiating of personal terms. Cahill told the club's official website, "Chelsea is a massive club, it is a club that looks to win trophies season in season out and it is a big opportunity for me to be a part of that. Opportunities like this you can't turn down." He also thanked his former club and their fans, saying, "This is the right move for me at the right time and I would like to thank everyone at Bolton because I've had a great four years at the football club. My spell with Wanderers has enabled me to break into the England squad, and I would particularly like to thank the fans for their support along the way. I always appreciated their backing." He was one of Chelsea's unused
substitutes in their 1–0 away victory in the
2011–12 FA Cup against
West London rivals Queens Park Rangers on 28 January 2012. On 5 February, Cahill made his debut for Chelsea in the 3–3 draw against Manchester United at
Stamford Bridge. On 18 March 2012, he scored his first goal in a Chelsea shirt against
Leicester City in the FA Cup, a 5–2 win. Cahill scored when he rose above
Wes Morgan to head the ball from six yards from a
Juan Mata corner, before unveiling the word's "PRAY 4 MUAMBA" on his inner T-shirt, in support of his former Bolton teammate
Fabrice Muamba, who had collapsed in a match the day before. He scored his first league goal for the club in the following match, opening the scoring in a 2–1 away defeat to
Manchester City. He made a substitute appearance in a 5–1 victory in the FA Cup semi-final against Tottenham Hotspur, after
David Luiz was taken off injured. Cahill started Chelsea's
UEFA Champions League semi-final first leg against
Barcelona because of the injury picked up by David Luiz in the FA Cup semi-final. The match finished 1–0, with Cahill making some crucial tackles in the second half. In the sixth minute of the second leg, Cahill stretched to win the ball and went down with a
hamstring injury. Although he tried to run it off, he eventually had to be replaced by
José Bosingwa in the 13th minute. Chelsea went on to win 3–2 on aggregate and set up a meeting with
Bayern Munich at the
Allianz Arena in the
2012 UEFA Champions League Final. Chelsea went on to win the season's FA Cup, beating Liverpool 2–1 at
Wembley Stadium in the
final, although Cahill missed the match with a hamstring injury. With the suspensions of both defenders
John Terry and
Branislav Ivanović, questions arose whether Cahill, along with David Luiz, would be fit for the UEFA Champions League Final to fill the gap. On 15 May 2012, he completed his first full training session after three weeks. "We've worked really hard, all day, afternoons, and I'm fortunate enough to be back," he said. "Hopefully, I'll be fit, I will be fit, and hopefully I'll be out there if selected." He started the match and was part of the squad that defeated Bayern on penalties in the 2012 UEFA Champions League Final, giving Chelsea their first ever
UEFA Champions League title. He received praise for his performance especially, as he was still suffering from the injury sustained during the semi-final's second leg. During his first half-season at Stamford Bridge, Cahill appeared in 19 matches, scoring two goals and providing one assist in 1,596 minutes on the pitch.
2012–13 season Cahill scored on his first appearance of the season in a 4–2 win against Reading on 22 August 2012. On 31 August, he scored in the
2012 UEFA Super Cup against
Atlético Madrid, which Chelsea lost 4–1. On 5 December 2012, Cahill scored his first UEFA Champions League goal, coming against
Nordsjælland in a 6–1 home rout. On 16 December 2012, Cahill was sent-off against
Corinthians in Chelsea's 1–0
2012 FIFA Club World Cup Final defeat after a clash with
Emerson. In January 2013, Cahill did not feature in the league match against Stoke City due to the birth of his son, Leo. On 15 May, he started the
2013 UEFA Europa League Final against
Benfica and made a vital last-ditch tackle in the final moments to ensure that Chelsea won the match 2–1.
2013–14 season Cahill made his season debut in
José Mourinho's first match back as Chelsea manager, helping them keep a clean sheet in a 2–0 victory over newly promoted
Hull City on 18 August 2013. Twelve days later, Cahill picked up his first yellow card of the season as Chelsea fell to Bayern Munich on penalty kicks after the teams played out a 2–2 draw in the
2013 UEFA Super Cup, held at the
Eden Arena in Prague. Cahill headed in Chelsea's equaliser in their 3–1 comeback win over
Southampton on 1 December 2013, moving within four points of
2013–14 Premier League leaders Arsenal. On 18 March 2014, Cahill scored on a rebound of a John Terry header, giving Chelsea a 2–0 win over
Galatasaray and a spot in the last eight of the
2013–14 UEFA Champions League. This match also marked Cahill's 100th appearance in a Chelsea shirt since his January 2012 move to Stamford Bridge. At the PFA Awards ceremony on 27 April 2014, Cahill was unveiled as a member of the
2013–14 PFA Team of the Year along with Chelsea teammates
Petr Čech and
Eden Hazard.
2014–15 season By 18 October 2014, Cahill was the only Chelsea player to play every minute of their 2014–15 campaign so far. On 8 November, he scored his first
Premier League goal of the season against
Liverpool, the equaliser in Chelsea's 2-1 win at
Anfield. On 24 January 2015, Cahill scored the opener against
Bradford City in an
FA Cup match, but in the end Chelsea
crashed out of the tournament after a 4–2 defeat, which ended their hopes of winning a quadruple this season. Cahill started for Chelsea as they beat
rivals Tottenham Hotspur 2–0 at Wembley Stadium in the
2015 League Cup Final. He won the Premier League title with Chelsea that season, as they finished first with 87 points. He was one of six Chelsea players named in the
2014–15 PFA Team of the Year.
2015–16 season On 2 August 2015, Cahill made his first appearance of the season in Chelsea's 1–0 defeat at Wembley Stadium in the
2015 FA Community Shield against
London rivals Arsenal. On 24 October 2015, Cahill scored his first goal of the season against West Ham United in a 2–1 defeat as Chelsea suffered their fifth defeat in ten matches. On 2 December 2015, Cahill signed a new four-year deal at Chelsea, keeping him at the club until 2019.
2016–17 season trophy Under new management in
Antonio Conte, Cahill continued his partnership with John Terry in defence at the start of the season. On 27 August 2016, Cahill made his 200th appearance for Chelsea at home against Burnley, helping his team to earn a clean-sheet for this first time during the season in a 3–0 victory. On 20 September 2016, with Terry injured and Ivanović being rested, Cahill
captained Chelsea for the first time that season, and scored his first goal of the season in a 4–2 win over Leicester City during the third round of the
EFL Cup. On 23 October 2016, Cahill continued to serve as captain with Terry on the bench. He scored his second goal of the season and the second goal of the match in a 4–0 win over former manager José Mourinho's Manchester United at Stamford Bridge. On 26 October 2016, Chelsea lost 2–1 to West Ham United in an
EFL Cup match, with Cahill scoring a consolation goal for the Blues in the 94th minute. On 31 December 2016, Cahill headed in Chelsea's first goal in a 4–2 Premier League win against Stoke City, in the process helping Chelsea to a 13th consecutive league win, a club record for the Blues. On 18 March 2017, Cahill scored the winning goal in a 2–1 away victory at Stoke City, having also scored in the reverse fixture against Stoke earlier in the season. The win sent Chelsea 13 points clear at the top of the table, with 10 fixtures remaining. On 20 April 2017, Cahill was named in the
PFA Team of the Year for the third time in his career. On 25 April 2017, Cahill scored Chelsea's second goal in a 4–2 Premier League win over Southampton, at Stamford Bridge, to help maintain Chelsea's seven-point lead at the top of the Premier League.
2017–18 season On 26 July 2017, Cahill was appointed as the Chelsea club captain ahead of the
2017–18 season, following John Terry's departure. On 12 August 2017, in the first match of Chelsea's 2017–18 season against Burnley at Stamford Bridge, Cahill was sent off in the 13th minute of the match for a reckless studs-up challenge on Burnley midfielder
Steven Defour, culminating in a sequence of events that saw Chelsea begin their title defence with a shock 3–2 defeat to a Burnley team that had only won once away from home in the previous season. Cahill was suspended for Chelsea's league matches against rivals Tottenham Hotspur, Everton and Leicester City. He captained Chelsea to victory at Wembley Stadium in the
2018 FA Cup Final, his team beating Manchester United 1–0.
2018–19 season After making just nine first-team appearances all season under new manager
Maurizio Sarri, Cahill confirmed he would leave Chelsea at the end of the campaign. He made his last appearance in the last home match of the season, coming on as an 88th-minute substitute against
Watford in the
Premier League, a game Chelsea won 3–0.
Crystal Palace Cahill signed for Premier League club Crystal Palace on 5 August 2019 on a two-year contract, re-uniting him with his former England manager
Roy Hodgson. He made his debut in an away 2–1 victory over Manchester United on 24 August. On 2 February 2021, he scored his first goal for Crystal Palace with the winner against
Newcastle United. On 1 August 2021, Cahill announced via social media that he had left the club following the expiration of his contract. He made 45 league appearances across two seasons for the club, scoring once.
AFC Bournemouth Cahill signed for Championship club
AFC Bournemouth on 20 August 2021 on a one-year contract. He was part of a defensive deployment conceding just twice in September 2021, winning him the club's Player of the Month award. Having not appeared because of a red card at the end of January 2022 and due to the loan signing of
Nathaniel Phillips, Cahill was released at the end of the season following a subsequent promotion to the Premier League. On 16 November 2022, he announced his retirement from professional football, ending an 18-year long career with over 450 league appearances. ==International career==