After months of agonising, the Roosters, NSW State of Origin and Australian Test captain confirmed his retirement at a stirring press conference at the SCG on Monday.

Less than a week after turning 29, Cordner became the third high-profile Roosters player to hang up the boots in 2021, joining club captain Jake Friend and veteran winger Brett Morris.

He said he finally made "my toughest decision" last week, having not played since sustaining another head knock during the 2020 Origin series opener last November.

"It just felt like it was a massive weight lifted off my shoulders," Cordner said.

"The last six months or so have been a really challenging time in my life ... all I ever wanted to do was get back and play the game that I love and most importantly for the club I love.

"I'm at a stage now where I'm doing really well. I'm fine, I'm back at training with the team full time and I was getting ready and prepared to play in a couple of weeks' time."

But while he felt fine training, Cordner admitted to having sleepless nights afterwards, knowing he wasn't quite right.

"Deep down I knew I wasn't a hundred per cent fully recovered," he said.

"The feeling of going out (and playing) and what the next head knock looked like to me, I couldn't get that out of my head.

"What I went through last year had a big impact on me and it's still plays a big part of me mentally where I was at.

"To be honest, I don't want to go back there.

"If I wasn't 100 per cent mentally, trying to go back and play this game, how tough it is and how fast it is at the moment, I'm kidding myself because if I ever did get another head knock, I don't think that I could have lived with myself.

"I couldn't get my head and my heart to align. I had to show the courage and be a man and do something about it."

In a measure of the respect in which Cordner is held in the game, a who's who of the Roosters and rugby league royalty attended his emotional announcement.

Club patriarch Nick Politis, dual premiership-winning great Cooper Cronk, fellow former teammate Anthony Minichiello, Blues coach Brad Fittler, the Tricolours' entire 2021 squad and even Cronulla captain Wade Graham was there.

Cordner, choking up on several occasions during his emotional 25-minute speech and press conference, thanked every one of them plus his family, who were also in attendance at the SCG, for helping shape his distinguished career.

One of the most reliable and fearless players in rugby league, Cordner figured in premierships with the Roosters in 2013, 2018 and 2019.

He led the Blues to Origin series victories in 2018 and 2019 and took over as Kangaroos Test captain in 2018.

Contracted for another two years with the Roosters, the back-rower will instead end with 181 games for the foundation club, 20 matches for the Kangaroos and 16 Origin games for NSW.

"As hard as it's going to be, because I feel like I've got a lot more left to give to the game and this club especially, I can look back and take peace from the game and the career that I've had at this club," he said.

"I can be very proud of what I've done."

Asked for a career highlight, Cordner said: "being a Rooster".

His retirement is another savage blow for the Roosters after Friend also quit mid-season because of concussion symptoms before scoring marvel Brett Morris last week hung up the boots with a knee injury.

Fellow representative stars Luke Keary and Lindsay Collins are also sidelined this season with long-term injuries.

Australian Associated Press