The left winger had one try in Souths' penultimate game of a forgettable year, touching down from what appeared a forward pass from Tallis Duncan pass at Accor Stadium on Thursday night.
"I don't think the ref was game to take it off him, to be honest," quipped Rabbitohs coach Wayne Bennett.
Johnston's 210th career try came in the final minutes before halftime and leaves the veteran only two four-pointers shy of equalling Ken Irvine's mark that has stood since 1973.
With Souths on the bye next week, the stage is set for Johnston to take a shot at history against his side's most bitter rivals, the Sydney Roosters, in the last round of the regular season.
Bennett downplayed the anticipation over a milestone that is fast approaching.
"I don't know why you're making such a fuss about it, to be honest," he said.
"It's there, it's not broken. The day it is, it'll be a great moment. But no one can predict when and where. So what, are we going to live the rest of our lives until it happens as, 'What's Alex going to do?'
"Let's just get on with our lives. That's what he's doing. It'll come and we'll all celebrate it."
The win finished as South Sydney's biggest of the year, and also their biggest over the Dragons either as a joint venture or as their predecessor club St George.
"They (Souths) kept doing their jobs and turning up and competing really well. That's the best part," Bennett said.
Johnston could've come even closer to rarefied air amid the carnage.
The winger tore past Hayden Buchanan and Corey Allan down the left in the first half but passed inside to Jye Gray for the Rabbitohs' second try.
After the break, Duncan had Johnston unmarked on his left but did not need the veteran as he crashed past Buchanan for a four-score lead.
From there, it was Johnston's teammates inflicting the pain on a Dragons side that, after a great month, suddenly looks very ready for the off-season.
Right winger Tyrone Munro put the icing on the cake by stepping around Clint Gutherson en route to an 85-metre try in the final minutes.
The victory guarantees the Rabbitohs cannot finish on the bottom of the ladder, after last week's win had all but confirmed Bennett would avoid the first wooden spoon of his premiership career.
The depleted Dragons had already been relying on a miracle to play finals but are now mathematically out of contention, left to rue errors and defensive frailty against the Bunnies.
"Souths had a shorter turnaround than us, they've got injuries (as well) so we're not tossing up any excuses," said coach Shane Flanagan.
"If I start tossing up excuses, then I become one of the weak ones.
"The opposition didn't give us a chance to breathe. They were outstanding with the football. We were the opposite."
