European champion Reilly had a spectacular run to finish ahead of Olympic champion Logan Martin of Australia in a wet and windy Glasgow.
Martin, the last man to go after the competition resumed following a rain delay, came up half a point short of Reilly's winning score of 95.80.
"It's the next level," said a jubilant Reilly.
"This is the biggest competition in the world outside the Olympics and this is the perfect stepping stone for me.
"I've won the Euros a couple of months ago and to have those two jerseys at the same time shows the hard work is paying off."
Having laid down a solid first run, Reilly landed everything he attempted in the second.
"That was pure and utter relief for me," said Reilly. "It's not as often as you'd like that you come off a course knowing you've done everything.
"To be one of the few guys still doing the double flair in competition, it's such a high-risk trick and it's taking that risk at the end of the run.
"I could have thrown it all away with that one trick but I'm glad I had that belief in myself and that it paid off."
American Nick Bruce finished third just ahead of former champion Rimu Nakamura of Japan.
France's former three-time European champion Anthony Jeanjean, who lost his continental title to Reilly in June, suffered another setback ahead of the 2024 Paris Olympics.
"It was an important step for me but I remain focused on Paris 2024 which is my priority."
