"And then you sing and you're like a lion", Cowell told her.
Courtney danced around on stage, belting out the lyrics, while everyone in the audience was shocked by the young performer. Howie Mandel told Courtney a story about Clive Davis discovering Janis Joplin.
Launching into Otis Redding's 'Hard to Handle', Courtney left the judges and everyone in the crowd completely speechless - jaws basically dropped everywhere.
"Bloody hell, Courtney", Simon Cowell said.
The schoolgirl was supporting music legends Sister Sledge at the annual event, which saw her manage to get the crowd on their feet during her energetic three-song set. "You're like this shy little thing when you first came out. I mean, genuinely incredible". "Oh my gosh! You are not from this era".
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"I can't sign you to a record deal. I'm Howie Mandel. The only thing I can do for you, young lady, is give you..."
Hadwin openly wept with joy as the golden confetti rained down from the rafters, and was embraced in a big hug by her proud father, who had been watching his daughter perform from the wings of the stage.
She tweeted her thanks Tuesday night.
A teenage singing star has sailed through to the live shows of the hit TV programme America's Got Talent after impressing a panel of celebrity judges.
Courtney is no stranger to talent shows - she also competed in The Voice Kids in 2017 and was mentored by McFly and McBusted star Danny Jones.