In June 2009, Cahill was called up to the England squad for the match against Kazakhstan; however, he only made his debut on 3 September 2010, coming on as a substitute against Bulgaria.

[8] Cahill started his career as central midfielder, but moved to centre-back after rapidly increasing in height within the space of a year when he was about 15 or 16, just when he was looking to get his first YTS contract.

[10] 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.

[14] 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.

[16] 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.

[22] Speculation surrounded Cahill's future in the January 2008 transfer window, with Birmingham City,[23] Bolton Wanderers and Turkish club Beşiktaş all reportedly showing an interest.

[26] 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.

[27] 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.

[30] 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.

[31] 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.

[32] 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.

[33] 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.

"[34] 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.

[38][39] 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.

[41][42] 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.

[54] Chelsea went on to win the season's FA Cup, beating Liverpool 2–1 at Wembley Stadium in the final,[55] although Cahill missed the match with a hamstring injury.

[56] 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.

[64] 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.

[67] 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.

[68] 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.

[69] 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.

[70] 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.

[78] 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.

[82] 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.

[85] 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.

[93] 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.

[95] 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.

[107] Cahill was called up to the England squad for the match against Kazakhstan on 4 June 2009 following the withdrawal of Rio Ferdinand,[7][108] but had to wait over a year for his first cap, until he came on as a second-half substitute for Michael Dawson on 3 September 2010 against Bulgaria in a 4–0 win at Wembley Stadium.

[112] On 2 September 2011, in his first competitive start, Cahill scored his first international goal for England against Bulgaria in the qualifying rounds of UEFA Euro 2012 in the 13th minute.

[126] He made one appearance in the tournament, in the dead rubber match against Belgium in Group G.[127] On 28 August 2018, Cahill "stepped aside" from the England national team, telling manager Gareth Southgate that he did not want to be considered for selection unless there was an injury crisis.