The Roosters were nearly the victims of a mighty upset on Saturday night and were made to fight for their victory at Polytec Stadium in Gosford, which keeps their unbeaten 2025 record (12-0) intact.
Cronulla, who finished the regular season fifth, dragged the minor premiers into the gutter and threatened to cause a boilover.
The Roosters led by 10 points with a little over 15 minutes left but when fullback Brydie Parker was sent to the bin for a flop, things unravelled.
Tries from Emma Verran and Chantay Kiria-Ratu in the space of four minutes pulled the Sharks level to set up a grandstand finish.
But in her 50th NRLW game, halfback Kelleher - deputising for the sidelined Tarryn Aiken - guided a 25m field goal over to send the Roosters into the decider.
"It's been a journey for Joss and she's had to control the team in the past and probably hasn't backed herself," said Roosters coach John Strange.
"For her to do what she did today with a field goal there in the last three minutes and the 40/30 in the second half ... she's really coming of age and I thought she was outstanding."
Strange's side will meet the winner of Sunday's preliminary final between Brisbane and Newcastle.
The Roosters blitzed all before them throughout the regular season and last lost a game in round five of the 2024 campaign.
But the Sharks at least provided something of a blueprint for the winner of the Broncos-Knights clash to consider if they want to wreck the Tricolours' perfect season at Accor Stadium next week.
Off the back of Kiria-Ratu's monster boot, Cronulla were able to hold their gloves up.
They gave away several penalties that invited attacks on their own line but backed their scramble and rush defence to shut down their opponents' star-studded backline.
Jayme Fressard put the Roosters ahead in the 10th minute opener but the Sharks hit back through Brooke Anderson.
At the other end, Cronulla held Eliza Lopamaua up and jolted a ball loose when Olivia Kernick looked certain to score.
Reigning Dally M medallist Kernick made amends just before halftime when she rampaged through the Sharks defence to put her side 10-6 up at the break.
The powerful lock then set up Macie Carlile four minutes into the second half and the Roosters looked to be cruising toward victory.
But Parker's flop threatened to bring everything undone as Verran and Kiria-Ratu pulled the Sharks level in quick succession.
Cronulla had all the running but Kelleher showed a calmness amid the chaos to step back and send the match-winning field goal between the posts in the 67th minute.
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