First known for her leading role on the series Intruders, she would later land a starring role as Eleven on the hit Netflix series Stranger Things. She was also cast in the 2019 blockbuster film Godzilla: King of the Monsters and as the titular character in the 2020 film Enola Holmes.
She was featured in a couple of episodes of Once Upon a Time in Wonderland as Young Alice.
When not acting, she's trained as a boxer. She guest-starred in episodes of Modern Family and Grey's Anatomy in 2015. She was featured in the music video for "Girls Like You" by Maroon 5 and she has performed Cardi B's part of the song at some of the band's shows. In 2019, she announced the release of her makeup and skin-care brand Florence by Mills.
She was born in Marbella, Spain, but her family moved to Bournemouth before she was five. She moved to Orlando, Florida in 2011. She has a brother named Charlie and sisters named Paige and Ava. She was in a relationship with Jacob Sartorius from late 2017 to mid-2018, then dated Romeo Beckham. In late 2019, she began dating rugby player Joseph Robinson.