[15][16] Cahill's mother, Sisifo, primarily has Samoan ancestry and was born in the small village of Tufuiopa, Apia, where his grandfather and great-grandfather held chief positions.

[12][17][18] His mother took on two jobs as a hotel employee and a factory worker to support the family financially after his father suffered a hip injury and was unable to work.

[19] Growing up in a soccer-loving family, Cahill developed a passion for the sport from an early age, playing constantly with his brothers, Sean and Chris, and watching Premier League matches on TV with his father, who is a West Ham fan from his upbringing in London.

[note 1][12][17] His father took him and his brothers, Sean and Chris, to soccer training in a local park every week, where Cahill developed a discipline and a perfectionist personality over time.

[15][24] Though his biggest influence came from his maternal grandmother, Asofa, who taught him the Samoan culture and beliefs of discipline, hard work, and perseverance – something Cahill would carry throughout his life.

[17] At the age of seven, Cahill started participating in private coaching under John Doyle, a retired Irish-Australian footballer, who he considered played a pivotal role in transforming his game.

[31] Measuring up to 1.65m in high school, teachers doubted Cahill's dream, saying he could not become a professional soccer player due to his small stature which he later said was the best thing he was ever told, saying, "To the people who said I couldn't do it, I say thank you.

[34] He began in their youth ranks and also became a ball boy for the first-team games, which fuelled his aspiration to play for the club's senior team.

Cahill became the youngest player at the age of 15 to ever play for the club, making his senior debut as a substitute and scoring his first goal with a header.

He scored the winner in the replay at Goodison Park, but chose not to celebrate his winning goal, stating, "I decided not to go mad and just pay my respects to the club that gave me my start in the game.

[60] During the second half of the 2008–09 season, Cahill was used more as the "utility man" due to his versatility as Everton was plagued by injuries to playmaker Mikel Arteta as well as strikers Victor Anichebe and Yakubu and some other key players.

[61] Owing to Phil Neville sustaining a knee ligament injury early in the 2009–10 season, Cahill was named as the replacement captain of Everton.

[64] Cahill dedicated his winning goal in a Europa League game against BATE Borisov to the victims of the earthquake and tsunami in Samoa, miming rowing a canoe after scoring in the second half of the match in Belarus.

[71][72] The run stretched to 34 games, but was finally ended when he scored the opening goal, a tap in from close range, in a 1–1 draw with Blackburn in January 2012.

[73] On 13 May 2012, during Everton's last match against Newcastle United at Goodison Park, Cahill was sent off for violent conduct after the full-time whistle for an altercation with Yohan Cabaye.

"[75] On 26 July 2012, Cahill signed for the New York Red Bulls of Major League Soccer as a Designated Player, for a fee reported to be approximately £1 million.

[78] On 20 October 2013, Cahill set a new record (broken in 2015 by Mike Grella) for the fastest goal in MLS history when he scored eight seconds into a 3–0 win against the Houston Dynamo.

[79] Cahill replaced suspended forward Bradley Wright-Phillips in the second leg of the Red Bulls' MLS Cup semi-final against the New England Revolution on 29 November 2014.

Despite Cahill opening the scoring in the 26th minute to level the tie, the Red Bulls bowed out with a 2–2 draw in Massachusetts, losing 4–3 on aggregate.

On 16 February 2016, Cahill announced on social media that he had terminated his contract with Shanghai by mutual agreement upon being told that he was not part of new coach Gregorio Manzano's plans for the 2016 Chinese Super League season.

[95] In his second game for Melbourne City, he scored and got an assist in a 4–1 win in the FFA Cup quarter final against Western Sydney Wanderers.

[98] On 4 February 2017, just as he was preparing to come on as a substitute against Melbourne Victory FC, Cahill was sent off for using "insulting, offensive, abusive language towards a match official" after he angrily disputed a goal with Chris Beath.

In February 2002, Cahill received an offer to play for the Republic of Ireland at the 2002 FIFA World Cup by Mick McCarthy, his former manager at Millwall.

FIFA changed its eligibility rules in 2003, allowing players capped at junior levels to switch international allegiance, meaning that Cahill was then able to play for England, Australia, the Republic of Ireland or Samoa.

Cahill played in the game as Australia beat Uruguay after a penalty shoot out to qualify for the World Cup for the first time in over 30 years.

[citation needed] Cahill was instrumental in Australia's qualification for the 2010 FIFA World Cup, scoring goals in crucial matches against Qatar and Japan.

[137] On 5 March 2014, in a pre-World Cup friendly match against Ecuador, Cahill became Australia's all-time top goalscorer with 31 goals, scoring twice in a 4–3 loss.

In May 2018, Cahill was named in the World Cup squad by coach Bert van Marwijk and made a single appearance in the tournament, coming off the bench in the 63rd minute of the 2–0 loss against Peru.

[144] On 16 July 2018, Cahill announced his retirement from international soccer,[145] subsequently overturned with the decision to participate in one final home friendly against Lebanon in November 2018.

[159] In 2024, Cahill was widely rumoured to be the new CEO of Malaysia's national football team, however, was named as the personal advisor to Tunku Ismail Idris, Regent of Johor, regarding matters of sports and international relations.