The 14-year-old star of the Netflix series "Stranger Things" said it was a "dream come true" to become an envoy for the children's fund, according to a statement released by UNICEF.
To mark her appointment on World Children's Day, Brown will visit the Empire State Building in New York to see the landmark lit up in blue.
Brown will be tasked with using her global platform to help raise awareness of children's rights and issues affecting youth, such as lack of education, safe places to play and learn, and the impact of violence, bullying and poverty.
Brown joins the ranks of famous faces who have acted as UNICEF ambassadors, including David Beckham, Orlando Bloom, Jackie Chan, Ricky Martin and Shakira.
"It's a huge honour to join such an impressive list of people who have supported UNICEF over the years," Brown said.
"I am looking forward to meeting as many children and young people as I can, hearing their stories, and speaking out on their behalf."
On Brown's claim of UNICEF title, Henrietta Fore, the executive director of the organization, expressed her pleasure to welcome Millie to the UNICEF.
"I know that Millie will use her passion and dedication to defend the rights of vulnerable children and young people everywhere," Fore said.
Universal Children's Day - celebrated on 20th November -- is a global day of action 'for children, by children' to raise awareness and funds for the millions of children that are unschooled, unprotected and uprooted.