A New South Wales State of Origin and Australia international representative centre, he played his entire National Rugby League career for the St. George Illawarra Dragons, with whom he won the 2010 NRL grand final.

He originally gained an interest in rugby league football as a supporter of Sydney's Western Suburbs Magpies, the club his father followed.

He began playing football at the age of six for his local junior club, the Shellharbour Sharks, while also participating in athletics and surf lifesaving.

At the age of twenty-one Cooper made his NRL début for the St. George Illawarra Dragons in the opening round of the 2000 season against local rivals the Cronulla-Sutherland Sharks.

[citation needed] Though all was not lost for Matt as he was nominated for the Dragons player of the year, eventually losing out to Ben Hornby while he picked up the Geoff Selby Memorial NRL Coaches Award.

][citation needed] Later Cooper was called up for the third and deciding game of the 2004 State of Origin series for New South Wales after the team had lost the second match, replacing Luke Lewis to partner his clubmate Mark Gasnier in the centres.

At the conclusion of the 2004 St. George Illawarra Dragons season Gasnier had scored the most tries, with a total of seventeen, and the international selectors assigned him a spot in the Australian team for the 2004 Kangaroo tour of Great Britain, France and the United States.

[2] His hamstring injury forced Cooper to withdraw from the starting line-up for the 2006 State of Origin series, being replaced by winger Eric Grothe, Jr. from the Parramatta Eels[2].

[6] In the first round of the 2010 finals, St. George Illawarra defeated the Manly-Warringah Sea Eagles 28–0, with Cooper scoring two tries, surpassing Nathan Blacklock's record of 100 tries to become the Dragons highest try scorer, with 102 tries.

On 3 October 2010, Cooper was a part of the St. George Illawarra team that beat the Sydney Roosters 32–8 in the 2010 NRL Grand Final, helping the joint venture club claim its first premiership.

The following February Cooper travelled with the St. George Illawarra to England to play the Super League champions, Wigan Warriors in the 2011 World Club Challenge.