The midfielder signed his first professional contract at the age of 18 with English Division Two side Millwall FC. His time with the Lions coincided with one of the club's most successful periods, with the club earning promotion to Division One for the 2001-02 season and advancing to the FA Cup Final in 2004. In all, Cahill made 241 appearances for the Lions in all competitions, scoring 58 goals.
2004 also saw Cahill earn his first cap for Australia, despite having played for Samoa at the u-17 level. He was a key member of Australia's campaign in the 2004 Summer Olympics, which, along with his performances for Millwall, earned him the 2004 Oceania Footballer of the Year award.
He transferred to Everton for the 2004-05 season in which the club finished in the top four at the expense of crosstown rivals Liverpool. The following season, he was named one of the 50 finalists for the 2006 Ballon d'Or (and was the only finalist named from the Asian Football Confederation and Oceania).
On 12 June 2006, Cahill came on as 53rd-minute substitute in Australia's opening 2006 World Cup group stage match against Japan. The Socceroos were down 0-1, but Cahill scored an equalizer in the 84th minute -- Australia's first-ever World Cup goal. He added another in the 89th minute before teammate John Aloisi contributed a third in stoppage time to claim the country's first World Cup win.
The Socceroos finished second in their group to advance to the Round of 16, where they lost to eventual winners Italy, 1-0.