The 17-year-old singer came in 12th place on The X Factor as part of the girl group Sweet Suspense, though the group has since broken up.
Millie went to see Demi perform during the opening night of the Future/Now Tour in Atlanta the other day.
“This lovely lady KILLED it tonight!! #TheFutureNowTour was incredible, but the highlight of the night was definitely seeing Demi backstage! She told me I’d grown up since #XFactor haha. Can’t wait till the next time I see her,” Millie captioned a photo of herself and the former judge. Go see it on her Instagram page!