In 2001, Cronk was selected to play rugby league for Queensland and Australian Schoolboys teams. He was then selected to play in the Queensland Cup’s Norths Devils team. He quickly established himself in that competition and trained with the Storm Squad in 2003. He scored 37 tries for the Norths Devils in the Queensland Cup.

Cronk made his debut for the Melbourne Storm in 2004, starting as a utility back coming off the bench and playing at five-eighth, hooker and lock.

In the 2006 NRL season, Cronk won the Dally M Halfback of the Year award. He also played at half-back for Melbourne in the 2006 NRL Grand Final loss to Brisbane.

In 2008, Cronk was called into the Australian test team to replace Jonathan Thurston who had twisted his ankle in training. Thurston later was cleared of injury and played in the test team over Cronk. In August, 2008, Cronk was named in the preliminary 46-man Kangaroos squad for the 2008 Rugby League World Cup, however he was not selected to make an appearance. When Cameron Smith was sidelined by a NRL Judiciary suspension during Melbourne’s finals campaign of the 2008 NRL season, Cronk stood in as captain and captained the Storm in the 2008 NRL Grand Final defeat by Manly Sea Eagles.

In April 2009, he was named in the preliminary 25-man squad to represent Queensland in the opening State of Origin match for 2009, however he was not selected to make an appearance. He played in the 2009 NRL Grand Final against the Parramatta Eels, setting up 2 tries. This title was later stripped due to salary cap breaches. Cronk was named as part of the Four Nations team and played for Australia in 2009 Four Nations Tournament in Australia against France as a utility.

Surgery on his groin saw Cronk miss Melbourne’s appearance in the 2010 World Club Challenge. An injury to Johnathan Thurston saw Cronk selected for the 2010 ANZAC Test. Cronk played off the bench as a utility for Queensland in the 2010 State of Origin Series, playing in all three games. In Game II, he scored his first Origin try.

He also played for Australia in the 2010 Four Nations.

Cronk played in the 2011 State of Origin Series. In Game I, he set up a try for Jharal Yow Yeh in the second half and in Game III, he set up Greg Inglis as well. In 2011, Cronk also played the tests against New Zealand at the Gold Coast and Newcastle on the bench. Cronk was part of the 2011 Rugby League Four Nations tournament, playing off the bench when the Kangaroos played the Kiwis and England. Cooper Cronk has been selected to play Five-Eighth against Wales, with Johnathan Thurston at halfback, resting Darren Lockyer for the Four Nations Final. In the 2011 Rugby League Four Nations between Australia and Wales, Cronk scored 3 tries and set-up 2 tries for Australia. He also played in the Four Nations Grand Final against England.

As of 29 March 2012 Cooper agreed to a further four years with the Melbourne Storm. At the 2012 Dally M Awards Cronk was named the NRL’s halfback of the year. In the 2012 NRL Grand Final victory over Canterbury-Bankstown, he won the Clive Churchill Medal for the man of the match award.

Cronk kicked the game and series-winning field goal in the final minutes of the third State of Origin match in 2012. He later said, “To be brutally honest I was in a state of grace at that particular moment. Every sinew in my body came together in one perfect whole. But those who have ever experienced that feeling, and it doesn’t happen very often, will tell you it’s in a whole other place of experience from the usual ego or vanity that drives my game. So I’m not afraid to own it for what it was.”

In February 2013, he won man of the match in Storm’s 18-14 win over Leeds in the world club challenge. Cronk was also selected as starting halfback for the annual ANZAC Test match between Australia and New Zealand in Canberra, playing at halfback for Australia. He also played a key role in Queensland’s eighth consecutive series win with a 12-10 win over New South Wales in the decider, setting up the match winner try to Justin Hodges.

Cronk broke his arm in the opening minutes of Game I of the 2014 State of Origin series on 28 May 2014. Written off for the series, he surprised the entire rugby league community by getting himself fit enough to take the field in Game III just six weeks later. Queensland lost the series after losing Games I and II, but in Game III on 9 July 2014 the Maroons routed New South Wales 32-8.

On 30 August 2014, Cronk played his 250th game for Melbourne Storm, a feat achieved by only 3 others players at the club.

2016 began with rumours linking him to rival NRL clubs in Sydney, most notably the St George Illawarra Dragons. However, he put that in the back of his head to lead Melbourne to a great start to the season. On the 2 April, he scored 2 tries to secure an 18-14 victory over the Newcastle Knights. 15 days later, he kicked a field goal in the 5th minute of golden point to give the Storm a 19-18 victory over the Wests Tigers. The Storm won 5 of their opening 7 games.

On 26 June 2016, Cronk became only the fifth NRL player in history to record 200 wins (from 289 games) in a 29 to 20 win over the West Tigers.

On 24 September 2016, in Melbourne’s grand final qualifier against the Canberra Raiders, Cronk became just the 25th NRL player to notch up 300 first grade games. In doing so, he is only the 11th player to do so with the one club.

At the annual Rugby League Dally M awards on 28 September 2016, Cooper Cronk was named the Dally M Player of the Year for a second time after winning his first in 2013. Cronk tied with North Queensland Cowboys prop Jason Taumalolo to win. Cronk was also crowned Dally M Halfback of the Year earning the honours for the fifth time in his careers. This was further complemented on 22 December 2016, when Cooper Cronk won the 2016 Rugby League World Golden Boot Award (Worlds best player). The award is decided by a panel of Rugby League experts including Darren Lockyer.

On 4 April 2017, Cronk made the announcement that the 2017 NRL season would be his last for the Melbourne Storm, stating he would move to Sydney; Cronk emphasised that this was purely based on family reasons rather than football-related reasons. His then-fiancee Tara Rushton, a Fox sports presenter based in Sydney, was the main driver behind his decision. On 30 October 2017 it was announced that he would join the Sydney Roosters for two seasons. Cronk and Rushton married on 14 December 2017.

In 2018 Cooper Cronk joined the Sydney Roosters. Where he played all 26 games, including playing with a broken scapula in the grand final victory over his former club the Melbourne Storm. He then became the first player to win consecutive premierships with two different clubs in the NRL era, having won the 2017 Grand Final with Melbourne and 2018 Grand Final with the Roosters starting at halfback in both games. Cooper also became the first player in the modern era to have three consecutive grand final appearances.

On the 6th of October 2019, Cooper Cronk played his final game of Rugby League in the NRL for the Sydney Roosters in the 2019 NRL Grand Final. Prior to his last match, the only thing he had not managed to achieve is back-to-back titles with the same club. He did that, and set the record for three consecutive grand finals in the modern era. Cronk was revered by the Rugby League community for his achievements within the game, to win six NRL Grand Finals from nine games.

On 7 November 2019, Cronk was recruited into the GWS Giants coaching staff as a ‘leadership consultant’. The move was initiated by Jimmy Bartel and was officially confirmed by their coach, Leon Cameron. Cronk will begin partial management in 2019 in preparation for the Giants’ 2020 AFL season.