After moving to Sydney in 2005, he began playing in the St. George Junior Rugby League Football Competition for Hurstville United Junior Rugby League Football Club.

King was a member of the St. George Dragons S. G. Ball Cup team in 2008. He debuted in round 1 against Parramatta Eels at Cabramatta Sports Ground on 16 February 2008.

King’s form resulted in him becoming a member of the St. George Illawarra Dragons under-20’s Toyota Cup team in 2008. He debuted in round 19 against Melbourne Storm at Olympic Park Stadium on 21 July 2008.

He attended and graduated from Endeavour Sports High School in Taren Point, New South Wales, where he captained the senior team and received the Endeavour Sports High School ‘Player of the Year Award’ in 2009.

While attending Endeavour Sports High School in 2009, King captained the Australian Schoolboys team which beat the Great Britain Community Lions team on two occasions with the scores of 66–0 and 66–18. King was named the most valuable player in each match and went on to win “Man of the Series” as well as the Great Britain Community Lions coaches award.

King played for Endeavour Sports High School in the 2009 ARL Schoolboy Cup.

In July 2009, he represented NSWCHS (NSW Combined High Schools) in the under-18’s division. He won the ‘man of the match’ in one game and was named MVP for the series.

In December 2009, he won the Val Lembit Memorial Trophy for the most outstanding sportsman in NSW, awarded by the Combined High Schools.

King made his first grade debut against the South Sydney Rabbitohs at Jubilee Oval in round 26 on 5 September 2010. The Dragons defeated the Rabbitohs 38–24 and won the J. J. Giltinan Shield in front of a sell-out crowd of 18,274 people.

King was selected to play for the 2010 Junior Kangaroos against the Junior Kiwis. The match was held at Westpac Stadium Wellington on 23 October. Australia won 24–16. King scored 1 try and kicked 2 goals. He then played in the second game of the series at Rotorua International Stadium on 31 October 2010. King converted two tries, scoring a total of 4 points. Australia lost 20–36.

On 13 January 2011 King was named in the “Blues in waiting” squad. A three-day camp from 21–23 January at Homebush was coordinated by coach Ricky Stuart for players who he believed had potential to feature in future New South Wales Origin teams.

On 1 February 2011 King was earmarked by The Daily Telegraph as a future NRL superstar in an article that also featured Dragons Toyota Cup teammates, Beau Henry and Kalifa Faifai-Loa.

King was given the opportunity to start at hooker for the annual Charity Shield match, against the South Sydney Rabbitohs, that was held at Stadium Australia on 13 February 2011, due to the absence of hooker Dean Young to injury and with Nathan Fien filling in at five eighth for Jamie Soward who was on representative duty for the annual All Stars Match.

On 22 February 2011 it was confirmed that King had re-signed with the Dragons for a further two years. In an interview with The Leader upon re-signing, King said “I’m happy here…It’s a good club to be at and I didn’t feel the need to look anywhere else.”

King was selected to play in the 2011 World Club Challenge on 27 February 2011 at DW Stadium against the 2010 Super League premiers, the Wigan Warriors. He came off the bench and in the 55th minute managed to dummy his marker from a metre out of the try line before barging over to score his first try for the Red V and one that would prove to be the game winner. Dragons went on to win the game 21–15 in front of 24,268 fans. He left the field shortly after scoring his try when he ruptured his pectoral muscle trying to tackle Warriors fullback Sam Tomkins.

On 5 August 2011, King made a successful comeback from the injury sustained during the World Cup Challenge by coming off the bench with a strong 50-minute performance in the Dragons under-20s 28–0 win against Wests Tigers at the Sydney Football Stadium in round 22. He went on to play the last 5 games of the season.

On 16 October 2011, King played for the Junior Kangaroos against the Junior Kiwis, at Ausgrid Stadium.

On 11 March 2012, the day after St. George Illawarra player Beau Scott had signed with the Newcastle Knights, it was reported by the media that ex Dragons coach Wayne Bennett was also attempting to poach King to the Knights. As Wayne Bennett got off the Knights’ team bus at Toyota Stadium as the Newcastle Knights were playing the Cronulla Sharks, an irate Dragons fan let him know what she thought of his poaching of Beau Scott and speculation that rookie hooker Cameron King may also be Newcastle-bound. “Don’t you take Cameron King off us,” the woman screamed as she poked her finger in Bennett’s face. “You’ve taken Beau Scott and Darius Boyd and all those others, but don’t you take Cameron King.”

On 12 April 2012, it was confirmed that King had re-signed with the Dragons for a further two years which will see him stay at the club until at least the end of the 2014 season. Upon re-signing, King said “It’s good. I’m really happy. I didn’t really want to leave and I’m happy to be here for another two years,’’ he said. ‘’It was the best thing for me and my family at the moment. I’ve got a lot of family in Sydney and to move anywhere else would have been hard.” “I’m really excited to have re-signed with the Dragons. We are a very proud club, I really enjoy being here and they have been great to me. We have a lot of young talented guys coming through the ranks and a great group of established senior players. I believe the club has a big future and I want to be a part of it.”

On 4 July 2012, King. along with teammates David Gower, Michael Lett and Jack Stockwell. played for the NSW Cup Representative side against the Queensland Residents at Suncorp Stadium. The match was a curtain raiser to the 2012 State of Origin game III decider.

On 25 September 2013, King signed with the North Queensland Cowboys. At the time King Said “The Cowboys have given me the chance to play some more regular NRL and an additional year so I am looking forward to this and am thankful to the Dragons to allow me the chance to take up this opportunity.”

On 13 August 2015, King signed a 1-year contract with the Parramatta Eels starting in 2016.

On 14 November, King took to Twitter in a desperate attempt to find a new club to play for in 2019. King posted Cameron King, Hooker, 90 kg, All round good guy, Enjoys hard work and winning, looking for a club, please RETWEET to raise awareness.

On 29 October 2019, it was revealed that King had been released by Featherstone. King spoke to the media saying “Things are probably taking a little longer than I’d hoped, But I do have options back in the UK and in Australia. It’s good to get back home and be talking to people and figure out what I want to do. I’ve been around friends and family and had a mental break from footy, but I’ve been assessing the options for next year, whether that be the Championship again, stepping up to Super League or coming back to Australia”.

On 2 December 2019, King signed a contract to join Cronulla-Sutherland for the 2020 NRL season.

On 7 August 2019, he declared his support for Leeds United.

In February 2020, King was taken from the field during Cronulla’s trial game against Canterbury-Bankstown with a suspected knee injury. It was later revealed that King had suffered a season ending ACL injury. King posted on Twitter saying “It’s still sinking in that my season is over. It’s been hard to accept what’s happened and I’d be lying if I said I’m ok. I’m taking things 1 day at a time and determined to not let this defeat me physically or mentally. Thank you all for the support”.

On 15 October 2020, King announced his retirement from the NRL.