The freeski slopestyle competition had been delayed by 24 hours because of poor weather in Colorado and it gives GB a third title after wins for Mia Brookes and Zoe Atkin.
“It’s an absolute dream come true,” said Muir.
“This morning I wanted to put that run down and to come out with gold, I’m so excited.
“I really wanted to improve [over the next two runs] but I had a couple of mistakes – but I’m so excited because I’ve missed X Games so much. To come here and win is insane.”
Muir was Team GB’s youngest competitor, aged 17, at the Beijing Winter Olympics in 2022 but missed a year of action from the end of 2023 after knee and shoulder surgery.