Boyle, who received a standing ovation as she took the stage, brought the entire studio audience and the judges to their feet.
The Spice Girl said: "What an absolute honour and pleasure it is to be sitting here and listening to you and your angelic voice".
The lineup for the night was Alex Magala, Bianca Ryan, Justice Crew, Preacher Lawson, Sara & Hero, Sofie Dossi, Susan Boyle, The Clairvoyants, Uzeyer Novruzov and Vicki Barbolak.
After her performance, Simon Cowell showered her with support, saying: "I can't think of any other contestant who has defined this show better if I'm being honest with you".
Boyle got world-wide attention back in 2009 when she wowed the audience with her powerful performance of "I Dreamed a Dream" on "Britain's Got Talent".
Trae Patton/NBCSusan Boyle is straight through to the live final of "America's Got Talent: The Champions".
Susan Boyle, who stunned audience members and judges almost a decade ago during her audition on "Britain's Got Talent", made a triumphant return to the stage Monday on "America's Got Talent: The Champions".
SuBo's television comeback has been hotly anticipated for months now and she shared a 'beautiful' moment with Simon Cowell when they reunited on the show.
Despite being the favourite to win the show back in 2009, Boyle lost out on the top spot to dance group "Diversity". However, all the attention catapulted her into the spotlight, where she released her debut album I Dreamed a Dream. The ones who people tend to ignore, I feel I'm a champion for them, she said. "You're the one. You made a huge difference to a lot of people's lives and I'm absolutely thrilled that you're here".
And with that, Susan Boyle is back.
Despite being a runner-up on the show, she went on to sell over 25 million records worldwide.