The 19-year-old was called into action for the second half of the second leg of a play-off with Clyde after first-choice Greg Paterson suffered a cut to his head.
It would prove to be an eventful debut, with Rooney making three saves during a penalty shoot-out to help preserve East Fife's status in SPFL League 1 for at least another week. They won the shoot-out 7-6 and will face Stirling Albion in the first leg of the play-off final on Wednesday night.
"I didn't expect any of that to happen," said Rooney. "As the substitute goalkeeper you are always ready to play but you do not expect to get on. It was an absolutely crazy day.
"Greg picked up a really nasty cut and he did really well to keep playing until half-time. That gave me the chance to get prepared."
He had not sat idle during the first half, with both technical areas filled with agitated players who sprang up at the slightest invitation. Rooney was on his feet in celebration as East Fife cancelled out Clyde's first-leg advantage early on when Scott McBride scored.
The visitors recovered their advantage before the break when Scott Ferguson got on the end of a long punt to poke a shot into the net. Then came Paterson's collision with Brian McQueen and a command for Rooney to make ready.
Kevin Smith's late goal helped take the tie to penalties and left Paterson centre-stage. Fraser McGhee missed from the spot to hand victory to the home side."I said to him before he took it that he was missing it," added Rooney. "I am glad he did."