In the final minute against Penrith, Sandon Smith intercepted a pass and raced 90 metres to seal Newcastle's win.
Earlier, his last-tackle run had cut the deficit to 14-10, and he later scored after a 50-metre sprint from a tap.
The 23-year-old will now become only the second Newcastle halfback to play in a grand final, after Andrew Johns.
The Knights rose from last year's wooden spoon to seventh this season and Sunday's decider.
Smith's own turnaround followed his departure from the Sydney Roosters, where he was effectively making way for Daly Cherry-Evans.
"They were transparent with me, and I've got no (ill-feelings) to the club," Smith said.
He joined Newcastle under former Roosters assistant Justin Holbrook but was not in the first-choice side until Dylan Brown ruptured his ACL in July.
Holbrook urged him to stay focused and improve, assuring him his opportunity would come.
Smith says Brown's advice on staying composed has helped him, while his play has created space for Kalyn Ponga and Fletcher Sharpe.
On Sunday, Smith will face his former club in the grand final.
"There's not a better one," Smith told AAP.
He said Newcastle believed it could win and that he was eager to face former teammates.
Smith called the season special and said, "And we've got one more to go, and we'll be the ultimate fairytale."