Millie Bobby Brown was born on February 19, 2004 in Marbella, Spain to English parents, Kelly and Robert Brown. Millie has an older sister, Paige, an older brother, Charlie, and a younger sister, Ava. The family moved back to England in 2008, and then to Orlando, Florida in 2011.

While living in Florida, Millie went to acting workshops to pass the time on a Saturday, and it was there that a top Hollywood talent scout called and told Millie's parents that "she has instincts you cannot teach." She advised Millie's parents that Millie could "mix it with the best kids in Hollywood." They packed up and drove from Orlando to Los Angeles, and within a week, Millie was meeting with the town's top children's talent agencies. She was offered representation by all the agents that she met. Within three months of being in Hollywood, Millie was offered the role of young Alice in ABC's Once Upon a Time in Wonderland. In November 2013, after just one self-taped audition, and without meeting the producers/directors, Millie was offered the role of Madison O'Donnell in BBC America's show, Intruders.

She then made guest appearances in CBS' police procedural drama NCIS, ABC's sitcom Modern Family, and ABC's medical drama series Grey's Anatomy.

Millie came to worldwide prominence for her role as Eleven in Netflix's science fiction mystery drama series Stranger Things. In November 2016, she starred in the music video for Sigma's single Find Me, featuring Birdy.