[citation needed] King began playing rugby league at the age of seven for his local club side the Casino RSM Cougars in northern New South Wales.

King suffered a serious shoulder injury in 2002 while playing off the bench for the Cronulla-Sutherland Sharks' reserve grade side and then quit rugby league because of a claimed disinterest.

[citation needed] After hanging up his boots at the age of 22, King took up several odd jobs such as working as a bar man and a garbage collector before deciding to give rugby league another go in 2003.

The 2004 NRL season saw King develop further as a player at the Storm, he cemented his place in the squad and was vital in the Melbourne side's attack, becoming a potent offensive weapon with his agility, pace, strength and ability to find the try line.

King finished the 2005 season at Melbourne with fourteen tries, and coupled with his three for New South Wales, earned a spot in the Australian international side for the 2005 tour of Great Britain.

After his side's 15–8 loss to the Brisbane Broncos in the 2006 NRL grand final, in which he scored a try, he cost his club $5,000 after publicly criticising match referee Paul Simpkins.

[9] After being consistently linked with a move to the Super League during the early half of 2007, Matt King finally announced on 10 July 2007, that he had signed a four-year deal with the Warrington Wolves.

In the semi-final of the 2009 Challenge Cup against Wigan Warriors, King scored a hat-trick of tries to help Warrington Wolves book a place in the Wembley final for the first time in nearly 20 years.

[19] On 29 June 2011 King announced that he would leave the Warrington Wolves at the end of 2011's Super League XVI to join up with departing Wigan Warriors coach, Michael Maguire at Russell Crowe's NRL team, the South Sydney Rabbitohs.

It was announced that he would captain the South Sydney Rabbitohs in the 2012 NRL season[citation needed] along with Michael Crocker, John Sutton, Roy Asotasi and Sam Burgess.

In 2015, King returned to the South Sydney Rabbitohs side for the Auckland Nines Competition scoring four tries, including the grand final winning try against Cronulla-Sutherland in extra time.