Johnston's third try, in the 70th minute, iced a back-and-forth game on Saturday afternoon at Cbus Super Stadium with the Titans slipping to their ninth defeat in 10 games.
The Souths winger brought up his 150th NRL try, becoming just the 15th player ever to achieve that feat and registered back-to-back hat-tricks for his side.
South Sydney's win continues their dominance over Gold Coast with their last loss coming six years ago.
Coach Jason Demetriou said his players managed their week off well and showed enough heart to stave off a spirited Titans outfit.
"It's always tricky when you come in after a bye and a week off to get that energy level where you want it to be straight away," he said.
"But credit the Titans, I think they completed at 90 per cent for the game, they came to play, their season's on the line and thought they were desperate and we had to work hard to get over them in the end."
Enjoying a 12-4 lead at the break, Johnston bagged his second just after halftime when some soft hands on the left edge put him into space and he carried enough momentum to the line through contact.
Titans skipper Tino Fa'asuamaleaui crossed to get them back into the contest, but Souths wrangled back momentum in the 60th minute after a stellar run by Campbell Graham was finished off by a surging Taane Milne who crashed over.
Fa'asuamaleaui bagged a late consolation minutes from fulltime but milestone man Tom Burgess still celebrated a win in his 200th NRL game.
Titans coach Justin Holbrook saw improvements from his side but inferred they lacked class with key players out.
"We just needed an extra spark out there," he said.
"We tried hard, we competed hard, the forwards were enormous ... but we just couldn't come up with a lot."
In the first half, the visitors enjoyed an eight-point buffer at the break after tries to Johnston and Jaxson Paulo.
They cracked the Titans defence after just six minutes when Cody Walker lofted a pass out wide to his winger who stepped off his left foot to score.
The Titans fired back through Greg Marzhew shortly after as he powered onto a long pass from Toby Sexton to find his way to the line.
Marzhew ran for 122 metres in the opening 40 but he couldn't stop Paulo minutes later when he latched onto a Lachlan Ilias high bomb, coming down through contact to score.
Paulo came close twice more but was stopped by scrambling defence in the final few minutes before Souths ran away with the contest in the second half.
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