The Light Blues shot- stopper was the hero for Gordon Durie's side as he saved four times in the shoot-out to break Hearts and win the Youth Cup for the Gers.
After Greg Pascazio and Craig Halkett had twice pulled Rangers level at St Mirren Park as the scores were tied at 2-2 after two hours of football, it was penalties that decided the destination of the silverware.
Kelly made a hat-trick of superb stops from the Jambos outfield players and when Calum Paterson made it 7-7, the goalie fired home from twelve yards to put the Light Blues ahead.
A terrific save from opposite number Jordan Millar sparked wild celebrations for Durie's stars and the spot-kick king was delighted his pre-match practice made for a perfect ending.
He said: "I was composed for the penalty. I think the keeper thought I was just going to go up and smash it.
"The easier option was to go to his left hand side. I showed a bit of composure and sent him the wrong way.
"I am a quiet guy, I just do what I am told. But when it comes to being on the park, I believe in my own ability.
"I would be daft to do otherwise. I wouldn't have been able to score the penalty or save the four that I did.
"There is no point standing on your line shivering and inviting the ball on top of you. You have to be commanding.
"I did practise them in training. I was going to go for more of a strike than a side foot. It would have been at the same side so he probably wouldn't have got there anyway.
"I do have sympathy for Jordan. I know him well. I never thought I would save four and he probably never thought he would save three.
"But, thankfully, I saved four and he saved three and we won."
It may have taken until the last kick of the ball for the tie to be decided but both teams had plenty of chances to claim the cup before the nerve-shredding shoot-out drama in Paisley, with striker Jamie Burrows enduring a night to forget in front of goal.
It took a late Halkett header, after Millar rushed from his line to claim a long ball, to keep the Gers' dreams alive and send the tie into extra-time.
And Kelly was delighted to end the season on a high note just days after the budding Light Blues had the Under-20 League title snatched away by Old Firm rivals Celtic.
He said: "It was eventful, to say the least, but it was enjoyable as well. It was a poor, poor first half from the boys to be honest but we got a rollicking at half-time, which we needed, from Gordon and Jim (Sinclair).
"In the second half, we had another gear and really started to apply the pressure to Hearts and thankfully we got the win.
"When you miss so many chances, you think it is not going to be your night.
"You need a bit of luck and a bit of fortune, it was a gamble that paid off."