Forty-eight-year-old Susan sang I Dreamed A Dream - the song which rocketed her to fame - and her new single, Rolling Stones cover Wild Horses on the final of America's Got Talent.
Wearing a simple elegant black gown, and accompanied by a full orchestra, the former church volunteer belted out the tracks.
An audience member said: "People were crying and cheering her after her performance."
Music critics lauded her performance, predicting a number one album and success in the US – the country which numerous British-based acts, including Robbie Williams and Kylie have found it difficult to crack.
A prerecorded interview with Susan revealed her new, positive attitude to singing after a tumultuous relationship with her new-found celebrity.
"I used to be a kind of spectator looking outward at the world," said the singer, who was mobbed by press and fans when she arrived in the country at the start of this week.
"But now I'm part of that world and, though frightening, I'm going to embrace it."