In front of 13, 873 fans who had to brave an electrical storm at BlueBet Stadium, St Helens became the first British side to win the competition on Australian soil since Wigan defeated Brisbane in 1994.
Halfback Lewis Dodd kicked the winning field goal to seal victory for the visitors.
Spearheaded by mercurial fullback Jack Welsby and veteran hooker James Roby, the Saints became just the third team to beat Ivan Cleary's men at BlueBet Stadium since the start of 2020.
They looked to be heading for victory inside 80 minutes until Brian To'o scored with 90 seconds left.
Nathan Cleary kicked the conversion to tie the game up but a knock-on from Stephen Crichton, who was standing in at fullback for Dylan Edwards, allowed Saints to strike through Dodd in extra time.
The Saints were more than a match for the Panthers.
They opened the scoring when Curtis Sironen made a bust upfield with Welsby flying up in support to go over after eight minutes.
Soon after, the livewire St Helens No.1 stopped Cleary scoring a certain try and his efforts allowed his side to go on the attack.
Penrith withstood some early pressure but there was nothing they could do when Konrad Hurrell barrelled his way over for the English champions in the 15th minute.
Penrith needed a circuit-breaker and it came in the shape of a trademark Cleary bomb, with Will Hopoate knocking on a floating kick.
Things got worse for the tourists when they lost winger and first-choice goalkicker Tommy Makinson to a HIA but they were able to repel a clunky Penrith attack to go in 10-0 up at halftime.
The resumption of play was delayed by 10 minutes due to the lightning but when the second half kicked off Saints were able to add a further two points through a Mark Percival penalty.
The Panthers needed a lifeline and it came in the shape of a smart chip kick from Cleary in the 50th minute.
Welsby flapped at the ball as it went into the St Helens in-goal and Izack Tago scored, with Cleary adding the conversion.
To'o then barged his way over at the death to send the game to golden point.
But it was St Helens' night with Dodd kicking the crucial field goal to send a travelling legion of barmy British fans into a frenzy.
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