The former ACT Brumbies star and 38-Test Wallaby will play for the NSW Waratahs next season after confirming a three-year deal on Sunday.
The 30-year-old has been with Top 14 outfit ASM Clermont Auvergne for two years after shifting from the Western Force in 2023.
He had previously served under McKellar in Canberra, with the experienced coach now pulling the strings at the Waratahs.
Fainga'a will effectively replace Dave Porecki, who retired suddenly this year.
Ahead of the 2027 showpiece in Australia, he'll add competition at Test level for Billy Pollard, Matt Faessler, Josh Nasser and Brandon Paenga-Amosa.
"The Waratahs is somewhere I started my professional rugby journey in the under-20s and to be coming back to work under Dan and play with some guys I know really well is something I'm massively looking forward to," Fainga'a said in a statement from the Waratahs."To be closer to family is something I'm grateful for as well after being away from them for nearly 10 years now."I feel I have plenty to offer on and off the field and am looking forward to working hard and giving myself every opportunity to make a positive impact on Australian rugby."
McKellar said it was a fine outcome after Porecki's "unexpected" retirement caught him by surprise.
"We needed to find someone of equal quality in a very short space of time, and we have done that," he said.
"We (at the Brumbies) recruited Folau from Sydney Uni in 2017 and saw a dynamic hooker with strong set piece.
"Back then we thought he was a player with the potential to grow and develop into a very good hooker and he has done that."
Fainga'a's signing comes as the Wallabies prepare to face New Zealand at Auckland's Eden Park on Saturday without key backrower Rob Valetini.
The Brumbies star was a late scratching from the touring party with a calf issue and has been replaced by the versatile Pete Samu.
Valetini, who has won the John Eales Medal as Australia's best Test player in the past two years, has struggled with injury this year and played just one Test during the British and Irish Lions series, missing the lead-in match against Fiji due to a calf injury.
The Wallabies, already without Will Skelton, have not won at Eden Park since 1986.
They will host New Zealand in Perth a week after the Auckland clash.
Chasing their first title in 10 years, Australia lead the Rugby Championship on 11 points, one ahead of second-placed South Africa and third-placed New Zealand with two rounds left.
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