Extra-time was required to separate the sides at the Penny Cars Stadium in Airdrie as Musselburgh edged past their opponents 3-2.
Tynecastle took the lead in the 11th minute. Charlie Nairn whipped the ball in from the left and although the keeper managed to get a hand to the ball to push it away from goal, it only got as far as Callum Todd and he slotted the ball into the bottom-right corner.
Musselburgh levelled before the break through a quick counter-attack. There were only five passes from the start of the play within the Musselburgh half to the ball hitting the net. Evan Fleming floated a ball into the box for Josh Murray to meet and head in at the near post.
At half-time, the even scoreline reflected the match as the two teams battled hard, but neither was able to stamp their authority on the match.
In the second half, Tynie created more goal-scoring chances but couldn’t find a way through.
Musselburgh the took the lead against the run of play in the 69th minute. Tyler Pryde fired a low ball into the box, a miss-kick from a Tynie defender sent it goal-ward and Robert Hunter followed it into the net.
As the game drew to a close, Tynie threw everything they had left at goal and Sam Nicholson became their hero. With virtually the last kick of regular time, his free-kick from 35 yards dipped into the net.
In the first half of extra time, Liam Henderson headed in from a corner to regain the lead for Musselburgh with what would prove to be the winning goal.
The full-time whistle sounded to send the Musselburgh support and bench into frenzied celebration. Tynecastle proved worthy finalists, but Musselburgh lifted the U-15s Scottish Cup for the first time since the 2004/05 season.
Longniddry Villa Under-16s battled bravely on Saturday morning at the Excelsior Stadium in Airdrie, but fell to Giffnock in the U-16s Inspiresport Scottish Cup.
Both teams came close to going ahead in the first five minutes, but the game soon settled with Longniddry looking particularly dangerous on the break and at corners.
The best chance of the half fell to Jack MacDonald, who had Nathan Muir in support but decided to go alone and his shot was saved.
Giffnock took a lead into the break through a Scott McMillan strike and, truthfully, neither side deserved to be down at half-time.
Aiden Walsh put in a man-of-the-match performance to try and get Villa back in the game, but his effort in the 51st minute flashed agonisingly across the face of goal and out of play. A minute later, Villa had a corner which was sent to the back post, but Ben Wright couldn’t steer his header towards goal.
Giffnock sealed the win at the death with a low shot from Ben MacFarlane that found the opposite corner.
Longniddry performed valiantly in poor weather conditions but ultimately were unable to replicate the heroics of last year when they were crowned Scottish Cup champions.