What happened in between was her transformation into a soulful singing dynamo that prompted judge Howie Mandel to compare her to Janis Joplin.
The teen performed a high-octane version of Otis Redding's Hard to Handle - seemingly totally at odds with her shy demeanour just moments before.
First she was on "Ellen", then on NBC's "Little Big Shots".
In her performance Tuesday, she wowed the judges with her rendition of Frank Sinatra's "My Way". "Oh my gosh! You are not from this era".
"I'm not Clyde Davis".
AGT•NBCAmerica's Got Talent The audience were visibly shocked when Courtney started singing
Fellow judge Howie Mandel was so impressed he hit the gold buzzer sending her through to the live shows.
"Thank you so much Howie Mandel I am so happy and can't thank you enough".
Courtney danced around on stage, belting out the lyrics, while everyone in the audience was shocked by the young performer.
America's Got Talent airs Tuesdays at 8/7c on NBC. Watch the video to see her unbelievable performance.
But despite missing out on the title, Courtney's performances on the show earned her a string of rave reviews from a new army of fans on social media.
And the young talent is no stranger to the limelight of TV, having already stared in the ITV's The Voice Kids a year ago.