The Queen asked Millie, who suffered injuries to her legs, if she had enjoyed the concert before the attack - prompting the teenager to reveal she had won two VIP passes and met the global superstar backstage.
The royal described the atrocity as "very alarming" and wished Millie, from Co Durham, a speedy recovery.
"It's not something you expect at all," the Queen said to father David, who was waiting at the exit of the arena for Millie when the bomb exploded.
She also spoke to Amy Barlow, 12, from Rawtenstall, Lancashire, and her mother, Kathy.
Queen Elizabeth II meets Amy Barlow, 12 and her mother, Kathy. Photo / AP
She met scores of staff who had worked through the night, shaking hands and sharing words with many, before visiting a ward where four young girls who were injured in the concert blast are recovering.
Twelve children under the age of 16 - among the 64 casualties - were taken to the hospital by ambulance following the terror attack.
Queen Elizabeth II speaks with hospital personnel as she visits the Royal Manchester Children's Hospital. Photo / AP
Prime Minister Theresa May visited the hospital on Tuesday afternoon.