After trailing 24-12 with 25 minutes to play, the Rabbitohs scored twice before Humphreys iced a late field goal to claim victory.
Error-riddled and on the back foot for the majority of the game, the Rabbitohs looked likely to fall victim to a Clint Gutherson attacking clinic at WIN Stadium on Saturday.
But in temperatures beyond 30 degrees the Dragons' defence wilted late, leaving Shane Flanagan's side winless after the opening two rounds.
The Rabbitohs scored less tries than the Dragons, had less time with the ball, ran less metres, missed more tackles and made more errors.
But still they came out winners, in a situation that has symbolised the start of Wayne Bennett's second stint at Redfern.
Long odds to even contend for the finals when Cameron Murray and Latrell Mitchell went down within moments of each other at training last month, South Sydney are now 2-0 to start 2025.
After leading 12-4 midway through the first half on Saturday, the Rabbitohs let the Dragons back into the match when they repeatedly failed to complete sets after points.
Gutherson was also putting on a masterclass, sending Tyrell Sloan over for two tries before half time and having a hand in two more after the break.
But when the Dragons looked set to run away with the match at 24-12, South Sydney recovered.
Cody Walker scored first on the left edge, before Sean Keppie got the Rabbitohs level with nine minutes to play.
And when Humphreys slotted his bizarre 76th-minute field goal, the Rabbitohs had the most unlikely lead.
With the Dragons defence flat-footed after Cody Walker dropped a ball backwards, Siliva Havilii passed the ball out the back to Humphreys.
Then, with virtually no defensive pressure on him at all, Humphreys was able to take his time to kick home a 30-metre field goal.
Humphreys was considered an unlikely option to wear the Rabbitohs' No.7 jersey this year, given the club's purchase of Lewis Dodd from England.
But while Dodd played reserve grade for the Rabbitohs on Saturday, it was Humphreys who came up trumps in the NRL.
On a day where they lost backrowers Tallis Duncan to a hamstring injury and Jai Arrow to repeated HIAs, the other positive for Bennett would have been hooker Peter Mamouzelos.
With Bennett admitting interest in Sydney Roosters hooker Brandon Smith, Mamouzelos had one of his best days in first grade.
The No.9 scored in the opening half when he strolled over from dummy-half, fooling Gutherson in defence and going himself.
He also helped lay on the crucial late try for Keppie, when he put the prop through a hole.
30°C