Even the show's judges admitted their surprise as fan favorite acts lost out to dark horse finalists.
But the feisty 14-year-old ended the competition in sixth place, even though her final performance of "River Deep, Mountain High" impressed the judges.
Hadwin had been among the favourites to win the talent competition show after judge Howie Mandel, 62, used his Golden Buzzer during auditions to advance her in the competition.
Hadwin was pitted against fellow singer and father-of-six Michael Ketterer, 40, for the last spot in the final five.
One of the reasons why Shin Lim is in our top three is not just because of his skills, but also because of his performances.
Vocalists Daniel Emmet and Glennis Grace were also losers this season, along with comedian Vicki Barbalok and Samuel J. Comroe, while trapeze artists Duo Transcend were brought down to earth.
Acrobatic dance group Zurcaroh finished second, and electric violinist Brian King Joseph was third.
However, it was not enough to secure her the $1million (£760,785) prize, with magician Shin Lim crowned the victor.
Simon added: "I always say the most important part of being a star is being remembered and that's what you do. You've got to show your personality, you've got to be a showman".
"I felt a reaction in the room tonight when he sang the Garth song, it was actually incredible, and yeah, we are going to be working together, there's no doubt", Cowell said. He will receive a US$1 million prize and headliner status for a November AGT show in Las Vegas.
Plus, Michael already has a big gig coming up: Garth Brooks invited him to perform at his concert at the University of Notre Dame on October 20!