Proceeds from the show, which will broadcast live across the globe, will go to an emergency fund set up by the city of Manchester and the British Red Cross.
The surprise visit comes less than two weeks after a suicide bombing that targeted fans as they left Grande's concert.
She sat at the bedside of children at the Royal Manchester Children's Hospital who are still recovering from the suicide bombing.
Lauren Thorpe, the mother of 8-year-old Lily Harrison, tweeted a picture of the pair hugging, writing: "Tonight's visit from @ArianaGrande was unbelievable". She has a big smile anyway but it got even bigger, Ariana gave her a hug and a kiss, she was in awe.
Mrs Mills, 42, added: "I think Evie is just buzzing from the visit last night and to go home and see her friends as well".
Other images show her hugging children in their beds and posing with nurses gathered on the ward. No1 deserves this more than her.
One Love Manchester will also air in countries including Brazil, Germany, China, Australia, Italy, Mexico and Sweden with more deals still being made. The gig is titled One Love Manchester and the USA superstar will be joined by a host of stars including Usher, Pharrell Williams, Justin Bieber, Little Mix, Coldplay, Niall Horan, Katy Perry and many more, with all funds going to the Manchester Emergency Fund.
According to Deadline, BBC One is airing the event in the United Kingdom and deals are being made in over 50 countries, including Brazil, Germany, China, Australia, Italy, Mexico and Sweden, to transmit the concert.
Grande and a host of celebrities are in the city to perform at a benefit concert on Sunday.
"So happy she came i could burst!", he wrote.