In temporarily moving back inside the top eight, the Rabbitohs added further misery to the Tigers' season as Johnston carved them up with three tries and an assist in Tamworth.
Souths didn't have it all their own way, conceding consecutive second-half tries to see a 20-6 lead evaporate, before late tries for Johnston and captain Cameron Murray sealed a much-needed victory.
But it was the return of No.1 Mitchell from a two-month absence with a calf injury that caught coach Jason Demetriou's eye, his side producing glimpses of the dangerous attacking football that has them rated a premiership contender.
Mitchell looked as silky as ever, running for 163m, busting seven tackles and offloading three times in an impressive outing that also included two vital defensive stops as the Tigers found their feet.
"He came up with some big plays and big moments, he's one of the best players in the game for a reason," Demetriou said.
"We saw glimpses of that tonight and the most exciting part is he's probably going to keep getting better which is good.
"We haven't played together, you can't just turn up, we go through a full pre-season of continuous reps, connection as a group, edges, spine, middle.
"The good thing is we're healthy now and we can build those connections over the next few weeks and we're excited about it."
But it's Johnston who'll remember his time in Tamworth more fondly, having broken the competition's record for most multiple-try matches with 51, passing Ken Irvine's mark.
Souths might not end the round inside the top eight but they're still a genuine contender, games against Cronulla (sixth), St George Illawarra (15th), Newcastle (10th) in the next three weeks ensuring they still harbour top-four hopes.
The Tigers entered the match on a seven-game losing streak but looked far better than that record suggested, captain Api Koroisau scoring one try and assisting another two as they gave the Rabbitohs a fright.
Johnston scored inside the opening two minutes after intercepting a loose John Bateman pass in a length-of-the-field effort before the Tigers fired back via Koroisau.
But it was the last time the Tigers got close in the first half, Johnson creating a try for Isaiah Tass via offload before the centre returned the favour soon after for a 14-6 buffer.
Rampaging second-rower Keaon Koloamatangi got over from a cheeky Lachie Ilias kick as Souths kicked into gear, but the Tigers wouldn't go down without a fight in what was their eighth straight defeat.
Tigers coach Tim Sheens said while their effort was never in question, a sloppy start littered with handling errors was costly.
"You can't give a side like that a 20-point start," he said.
"First half (we made) mistakes, a 58 per cent completion, I hate talking that sort of stuff but in real terms, the games we've won this year we were at 80 or 90 per cent.
"We were giving too much footy away, we're not that good a team to be able to give that away."
