UNICEF Goodwill Ambassador has become the youngest ever UNICEF Goodwill Ambassador after working with the children's charity for two years.
The 'Stranger Things' actress - who is best known for starring as Eleven in the award-winning Netflix sci-fi drama series - has supported the children's charity since 2016, when she hosted its 70th anniversary celebrations at the United Nations.
As a Goodwill Ambassador, the 14-year-old star will use her global platform to raise awareness of children's rights and issues affecting young people, such as poor education, violence, bullying and poverty.
Alongside a video explaining her role, she wrote on Instagram: "A dream come true! It's a huge honour to become @UNICEF's newest and youngest ever Goodwill Ambassador to help defend children's rights across the world. I can't wait to get started!
#WorldChildrensDay #GoBlue (sic)"
In a statement, she added:
"It's a huge honour to join such an impressive list of people who have supported UNICEF over the years.
"I am looking forward to meeting as many children and young people as I can, hearing their stories, and speaking out on their behalf."
The announcement was made on World Children's Day - 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".
Members of the public are encouraged to go online to sign a global petition calling on leaders to bring forth change to the lives of the millions of disadvantaged children on the planet.
UNICEF's Executive Director, Henrietta Fore, commented: "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."
Millie follows in the footsteps of the likes of David Beckham, Orlando Bloom, Jackie Chan, Danny Glover, Ricky Martin, Liam Neeson, Shakira and Lilly Singh in landing the role.
Last year, Millie induced 'Baywatch' star Priyanka Chopra into the "global family" alongside retired soccer ace David.