They gave coach Dave Howett and their band of supporters a few anxious moments late in the game, but at the full time siren the Jersey Flegg Sharks were celebrating a preliminary final victory as they moved into the under-20's decider next weekend.
One try just before the break and two more immediately after saw the Sharks skip out to a 16-point lead, however in the end they were forced to defend grimly to hold off the fast-finishing Mounties side in claiming a heart stopping 26-22 victory.
The Sharks 20's now move into a Grand Final match-up with the Panthers after the young Penrith outfit had scored late to edge out Newcastle in the earlier finals match.
The Mounties scored early before the Sharks would click into gear, a try to Issac Lumelume on the end of a quick backline shift, then a second to Harrison Smith when the hooker dived over under the posts, saw the Cronulla team take a 10-6 lead.
Mounties, who are the Jersey Flegg partner club for the Canberra Raiders, would level up with a try with five minutes remaining until the break, but it would be the Sharks to go to sheds with their tails up when fullback Luke Polselli split the defence and dashed 10-metes to score wide out.
Braydon Trindall was off-target in the windy conditions as the Sharks took a 14-10 lead to half time.
Encouraged by their try before the break the Sharks kept the pressure on their opponents in the early exchanges of the second period and it was Trindall with a show and go touching down just two minutes into the half. The young playmaker converted his own try to push the score out to 20-10.
Only three minutes later the Sharks were in again, Jackson Ferris after recently signing a senior-squad contract with the Cronulla club, scoring his 15th four-pointer of the season.
Again Trindall added the extras and the Sharks had a handy 16-point advantage.
It seemed as if coach Howlett and his staff could then sit back and relax, but that was far from the way it panned out, Mounties with their season on the line battling back to make a game of it.
Two tries in the last 10 minutes brought the margin back to just four however it wasn't to be enough, the Sharks showing enough resolve and determination to hold on and win their way through to the Grand Final next weekend.
They play the Panthers who managed to get the better of the Sharks by 18-10 in week one of the finals.
Stay tuned to the Sharks website for kick off times and details for the Flegg Grand Final which is scheduled to be played at Leichhardt Oval next Sunday.
Jackson Ferris, Braydon Trindall, Isaac Lumelume, Harrison Smith, Luke Polselli tries, Trindall 3 goals beat