Bieber showed up in the 7-year-old leukemia patient's room at Primary Children's Medical Center before his concert Saturday night.
Her mother, Amanda Flamm, told the Deseret News (http://bit.ly/SdSGQO) that Millie's mood brightened when Bieber walked into the room and gave her a big hug.
"He was so sweet to her," she said. "He walked in and her whole face just lit up."
Bieber sang a song while holding her hand and gave her his guitar pick.
"She is squeezing on to it with all her might and will not let it go," the mother said.
Bieber gave Millie a kiss on the cheek before leaving, prompting the girl to tell her mother, "You are never going to wash my face again."
Family friends launched an online campaign promoting the visit when a crushed Millie was forced to sell her concert tickets after suffering a relapse two weeks ago.
The family was not even sure Millie could meet Bieber because she had been running a fever and was on oxygen.
The visit will help Millie as she continues her cancer treatment, her mother said.
"It just has re-energized all of us," she told the Deseret News. "We are ready to keep going and fight harder with all this backing and this fun thing that has happened, and keep going and keep fighting.
Information from: Deseret News, http://www.deseretnews.com