Keary's much-publicised deal for 2025 was finally completed on Tuesday (Wednesday AEDT), as the Roosters wrapped up their camp in Los Angeles.
The 32-year-old's retention offers at least some stability for the Roosters, given the club is poised to enter a period of rapid transition over the coming few seasons.
In 2020 Keary signed a deal until the end of 2024, and had thought that would be his last contract in the NRL.
But he is now confident he still has more to give, and will play on until at least 2025.
"I did three years a while ago, and in my mind I thought that would be a full stop," Keary said.
"But as I got closer to the end, you start to realise you have a lot more left in you mentally and physically.
"I wanted to see if I wanted to do it, but my family too. I have three kids and my wife. But I wanted to check in with the club too and see the future they wanted to go."
The Roosters will lose Joseph-Aukuso Suaalii at the end of this season to rugby, and Jared Waerea-Hargreaves is poised to head to the Super League as the club's most-capped player.
Angus Crichton is also tipped to pursue a future in the 15-man game, while coach Trent Robinson has admitted he is resigned to losing Joey Manu to French rugby.
"He (Manu) is such a good person. He's been here for a long time, grew up here," Keary said.
"He has mentored a lot of these young kids coming through. I have seen him grow since he first came into first grade.
"He's a very good person and very good rugby league player too.
"If he does end up going, it's totally fine. We'll miss him a heap, but we'll watch his journey, and hopefully he comes back one day too."
The Roosters will leave Los Angeles for Las Vegas on Wednesday (Thursday AEDT), ahead of Saturday's clash with Brisbane in the historic season opener.
They are desperate to shake their tag as slow starters, having started favourites for every round-one match since 2018 but only winning one of them.
"We're due for a win," Keary quipped.
"We've had different pre-seasons in the past, some where we have travelled (for World Club Challenge), some where we haven't had guys train for a lot of it.
"We have had a consistent one this year, so hopefully we come out a little bit better than in previous years."
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