Competing against nine other acts, the Scottish singer stunned the crowd with her rendition of The Rolling Stones' 1971 hit "Wild Horses".
Presenter Terry Crews and judge Heidi Klum have both sung the 57-year-old I Dreamed A Dream singer's praises on Twitter, getting us even more excited to see her perform once again in the upcoming finals.
"It's great to be back", Boyle, declared just before she was given the stage to impress the audience with her voice.
"I just want to say what an absolute honor and pleasure it is to be sitting here and listening to you".
"I'm very happy and very humble", Boyle said of the accolades following her performance.
"Susan, I'm genuinely beyond thrilled that you're here tonight", Cowell told Boyle.
She is appearing on America's Got Talent: The Champions, alongside British teenager Courtney Hadwin, and made her debut on Monday night's show.More news: China: Man wounds 20 primary school children in Beijing attack
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Despite being the favourite to win the show back in 2009, Boyle lost out on the top spot to dance group "Diversity".
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.
The judge and former Spice Girl told Boyle she had "an angelic voice" and said she was giving her "what she deserves" as she slammed the Golden Buzzer.
Susan first appeared on Britain's Got Talent back in 2009 and surprised the audience with her rendition of I Dreamed A Dream.
Still, Boyle says she has "something to prove" on The Champions, and calls herself a champion "for those who don't have the confidence to do things, for those who don't have a voice (and) the ones who people tend to ignore".