She tweeted her thanks Tuesday night.
Launching into Otis Redding's 'Hard to Handle', Courtney left the judges and everyone in the crowd completely speechless - jaws basically dropped everywhere. Before she was even done, the audience was on their feet.
"Bloody hell, Courtney. You're like this shy little thing when you first came out".
Howie was so impressed by Courtney's performance that he said she was beyond this "era" - and compared her to an iconic singer! "And then you sing and you're like a lion", Cowell told her.
Fellow judge Howie Mandel was also blown away by Courtney, describing her as being like someone from a different era, before he pressed the Golden Buzzer - to signal that she has made through to the live shows in the competition. "I mean, genuinely, incredible".
This wasn't Courtney's first time performing on television.
Courtney said getting the golden buzzer was more than she "could have dreamed".
Watch her performance on the video below. "I'm not Clive Davis".
Courtney competed in The Voice Kids UK previous year but missed out on the top prize.
This week her audition from America's Got Talent went on TV and the result was pretty wonderful!