It was the first time Mel, 44, and Jamie, 15, had ever performed together in public. But that didn't stop the duo from Bridgend, south Wales, from reducing some of the talent show's audience to tears.
Watch their performance here:
Their audition aired in the United Kingdom on Saturday night and the footage is now going viral.
Head judge Simon Cowell said he initially feared Mel, who sings in residential care facilities for the elderly for her living, would "completely dominate" the song.
"(But) you were very cool in the way that you let Jamie take the lead," added Cowell, who is more usually known for his snarky put-downs. "You allowed it to be his song, but your harmonies actually improved the song."
The duo now joins 44 other acts for the live semifinals, which begin Sunday.
Other contestants include 14-year-old Jasmine Elcock, who brought the house down with her own twist on Cher's "Believe," the "Boogie Storm" dancing Stormtroopers and Beau Dermott, who blew judges away by singing "Defying Gravity."
See how Mel and Jamie's version compares to A Great Big World's original (before the version re-recorded with Christina Aguilera) here: