The news was announced on World Children’s Day (November 20), with the 14-year-old calling it “a dream come true”.
“It’s a huge honour to join such an impressive list of people who have supported Unicef over the years,” she 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.”
Brown – who rose to prominence following the break-out success of Netflix series Stranger Things – will use 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.
The actor has supported the organisation’s work since 2016, hosting its 70th anniversary celebrations at the United Nations in 2016, and championing its inaugural World Children’s Day last year.
She joins a list of Unicef Goodwill Ambassadors including Muzoon Almellehan, David Beckham, Orlando Bloom, Jackie Chan, Priyanka Chopra, Danny Glover, Ricky Martin, Leo Messi, Liam Neeson, Shakira and Lilly Singh.
Unicef executive director Henrietta Fore said: “I’m delighted to welcome Millie to the Unicef family, particularly on World Children’s Day – a day that is all about giving children and young people a voice.
“Children are their own best advocates. I know that Millie will use her passion and dedication to defend the rights of vulnerable children and young people everywhere.”
World Children’s Day 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.
Unicef is inviting the public to go online and sign a global petition asking leaders to commit to fulfilling the rights of every child now and for future generations.
The organisation is also asking supporters to help “turn the world blue” by doing or wearing something blue on November 20.
To kick-start the activation, last week Brown starred in a short video encouraging supporters to £GoBlue in support of children’s rights.
Other events include iconic landmarks such as Sydney Opera House being lit in blue, children taking over parliaments in countries around the world and youngsters going into newsrooms to report on issues that matter to them.
Several countries will also appoint Unicef Youth Ambassadors.
Additional reporting by agencies.