Winger Johnston carved the Tigers up in Tamworth on Friday night, scoring three tries and assisting another as the Rabbitohs at least temporarily moved back inside the top eight.
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.
No.1 Mitchell looked as silky as ever in his return from a two-month absence with a calf injury and played the entire game, running for 163m, busting seven tackles and offloading three times in an impressive outing.
He also came up with two vital defensive stops as the Tigers found their feet in the second half before the Rabbitohs steadied.
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 eight straight defeat.
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