The Broncos coughed up a 14-point half-time lead on Friday night and lost Ben Hunt to a hamstring injury, so may be forced to recall Ezra Mam next week.
After kicking the penalty goal that locked the scores 14-14, Mitchell put Souths on the path to victory by slotting a brilliant two-point field goal with three minutes remaining in Sydney.
"I don't think we thought it was going over there but all of a sudden we got excited," said Souths coach Wayne Bennett.
Mitchell rubbed salt into the Broncos' wounds by crashing over from dummy half after the siren sounded on his first game back from suspension.
"He just does what Latrell can do. That's what makes him such a special player," Bennett said of his star fullback's performance.
Souths' win snaps a four-game losing streak and breathes life into an injury-plagued campaign that had spiralled into the bottom four on the ladder.
Mitchell's performance has surely made his State of Origin case irresistible as well, with Queenslander Bennett dreading the possibility of Mitchell lining up for NSW come May 28.
"I hope they don't (pick him) ... he's such a threat. They (the Maroons) will all sleep better if he's not playing for NSW," Bennett said.
Hunt limped straight up the tunnel after pulling up sore on a kick chase in the second half, with his injury a chance to end Mam's exile next week.
Broncos coach Michael Maguire previously refused to put a timeline on Mam's return following his ban for drug-driving.
The coach may have little choice other than to recall the five-eighth for next week's clash against St George Illawarra given his replacement's injury.
Hunt's hamstring issue has also thrown his Queensland selection chances into doubt, with the veteran to go for scans on the weekend.
Maguire was ropeable with the Broncos at full-time, questioning whether they had the desire to play complete performances.
"Just the want to do it for 80 minutes," he said.
"We're chasing 80-minute games every week. We've seen signs but we've also seen the other.Â
"As a team they've got to learn how to do that or we'll keep bumbling the same way we have been. It's not good enough. Simple as that."
Souths had conceded at least 24 points in each of their four-straight losses but had renewed defensive vigour at Accor Stadium.
A lot of that was down to Mitchell, who pulled off two try-saving one-on-one tackles in the backfield.
He first stopped a rampaging Payne Haas on the prop's 40-metre tear down the left side.
Mitchell then dragged Deine Mariner into touch when the winger looked to make it a two-score game in the second half.
Mariner had put the Broncos up by 14 and deflated the Rabbitohs in the minutes before half-time following a shift down the right.
But Souths fired back only minutes after the break when Siliva Havili burrowed past three defenders from dummy half.
The margin was only two points when Isaiah Tass bulldozed through Kotoni Staggs ahead of his try down the left side.
Mitchell slotted a simple penalty goal with 10 minutes to play after Xavier Willison hit Lachie Hubner with a high shot, before going on to seal the most unlikely of wins.
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