Boyle, 57, chose to perform "Wild Horses", the same song she sang during her first-ever America's Got Talent appearance in 2009.
After hearing her perform, Mel explained why she had to press her golden buzzer.
When the television production saw Susan walk onto the stage, he couldn't help but express his joy: "Genuinely, I'm beyond thrilled that you're here tonight".
She then got a standing ovation from the audience for her performance.
The Scottish star made it to the final but came in second second place to Ashley Banjo's dance crew Diversity.
"I just want to say what an absolute honor and pleasure it is to be sitting here and listening to you".
The audience rose to their feet midway through her performance, which reminded us why we fell in love with Susan Boyle nine years ago.
Pleased that she had returned to the franchise that made her a household name, Cowell added: "I can not think of any other contestant who has defined this show better than you".
While Susan was the fan favorite to take home the win, she placed second overall.
Meanwhile, a source recently told The Mirror Susan's America's Got Talent return is just the start of her music comeback. "And I want to be the woman who gives you something you deserve".
"The anniversary of her audition which made headlines around the world is the ideal time to launch a career comeback, and Simon and Susan are both very excited about the year ahead". Despite not being an official champion, Boyle proudly said, "For those who maybe don't have the confidence to do things, for those who maybe don't have a voice, the ones who people tend to ignore, I feel I'm a champion for them".
Despite being a runner-up on the show, she went on to sell over 25 million records worldwide.
Susan's debut studio album, I Dreamed a Dream, was the UK's best-selling debut album of all time.
And with that, Susan Boyle is back.
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