Boyle received the show's highest accolade when judge Mel B pressed her golden buzzer to put the singer straight through to the finals.
A third wrote: "I pray you win the champion".
Boyle sang a cover of The Rolling Stones song "Wild Horses".
"What an absolute honour and pleasure it is to be sitting here and listening to you and your angelic voice", she said.
"I'm very happy and very humble", Boyle said of the accolades following her performance.
"I can't think of any other contestant who has defined this show better than you if I'm being honest with you", Simon Cowell told Boyle.
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She is appearing on America's Got Talent: The Champions, alongside British teenager Courtney Hadwin, and made her debut on Monday night's show.
Despite not winning it all, she credits the show with helping her "fulfil the dream I've had since I was 5", and in the process selling over 20 million records and becoming a household name.
Adding a sequin dress to the mix, Susan showed off her jaw-dropping style to the viewers as she shocked the audience.
Her first audition, a rendition of I Dreamed a Dream from Les Miserables, famously stunned the audience and judges alike.
She was taking part in the all-star version of the show, America's Got Talent: The Champions.
"You made a huge difference to a lot of people's lives and I am absolutely thrilled you are here".
An insider told the publication: "This is a big year for Susan and she is very much stepping back into the limelight with her AGT champions appearance being just the start. I want to be the woman that gives you something that you deserve", Mel B said before slamming the golden buzzer. She has released a further six studio albums, and been nominated for two Grammy Awards.