The older Mitchell brother's two-year contract extension moves him to the Rabbitohs' top-30 squad until at least the end of the 2024 season.
Mitchell, 26, made his first grade debut off the bench in round eight of 2022, having shed significant weight over the summer in the pursuit of his NRL dream.
He finished the year with two more NRL appearances to his name and played alongside his brother for the first time in the Rabbitohs' round-17 defeat of Newcastle.
"I'm very happy to be here for the next two years. South Sydney feels like home to me," Mitchell said.
"Souths gave me the chance to make my first grade debut which I will never forget and I can see a way forward here at this club.
"This contract allows me and my little family to have some stability, I can look after my three little ones, and I can concentrate completely on my footy."
Rabbitohs head of football Mark Ellison is confident Mitchell can challenge the likes of Junior Tatola, Thomas Burgess and Hame Sele for chances in the middle next season.
"Shaq's best football is certainly in front of him and his commitment to the club and his body shows he has the maturity to play more NRL at the Rabbitohs this coming season and next," he said.
The Rabbitohs now turn their attention to keeping marquee men Latrell Mitchell, Damien Cook and Cody Walker under the one roof beyond 2023.
The trio have been able to negotiate with rival clubs for 2024 since November 1 but have previously expressed their desire to remain together at Redfern.
All three command hefty pay-cheques but increases to the salary cap expected to be announced as part of the new collective bargaining agreement could help the Rabbitohs' bid to retain their stars.
The NRL and RLPA are currently brokering the terms of the new CBA, which will cover seasons 2023-2027, with details expected imminently.
Elsewhere, Manly have re-signed prop Morgan Boyle in preparation for the arrival of new head coach Anthony Seibold.
Boyle has played 48 NRL games across stints with Gold Coast and the Sea Eagles but had been without a contract for 2023.
With his new one-year deal, the 26-year-old joins Jake Trobjevic, Sean Keppie, Taniela Paseka and Josh Aloiai as an option in the middle for next season.
"We are hoping Morgan enjoys a better run of fortune next year as injuries have really hampered his progression in recent seasons," Sea Eagles chief executive Tony Mestrov said.
"He is a quality player, so let's hope we can see the best of him next year."
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