Already signed until the end of next season, Johnston will now remain at the club until at least the end of 2025 and past his 30th birthday.
It's a remarkable turnaround given the Rabbitohs were prepared to let him go just last year, before bowing to pressure and offering him a two-year deal.
The new deal means he cannot be a target for the Dolphins, after it appeared he might given his close relationship with Wayne Bennett and the fact he was off contract in 2022.
"I joked with 'Ello' (head of football Mark Ellison) that I wanted to be the first to re-sign this time around and it's great to have it all sorted out nice and early," Johnston said.
The deal also opens up a number of try-scoring possibilities for the 26-year-old who dotted down 30 times in 2021.
Johnston is third on Souths' all-time try-scoring lists, with 136 in 166 games.
Given his ability to find the line, he is poised to overtake the great Benny Wearing (144) and Nathan Merritt (146) next season.
Billy Slater holds the all-time record for the most tries at one club with 190 for Melbourne, a record that will be in Johnston's sight if he remains fit.
He would also require just 16 tries over each of the four remaining seasons on his contract to reach 200 tries.
Beyond that, Ken Irvine's all-time record of 212 tries remains in touch, particularly if he is to extend his career beyond the current deal.
"Alex is a tremendous finisher and is really building the other areas of wing play such as kick returns and defence," Ellison said.
"We're very excited to be able to extend his contract for a further three years on top of his existing contract.
"And we look forward to seeing him score many more tries in that left corner in the years to come."
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