Brown lahir di [[Marbella|Marbella, Andalusia]], Spanyol,<ref>{{cite web|url=https://www.youtube.com/watch?v=Da4SABaYXRs&feature=youtu.be&t=174|title=Millie Bobby Brown - Full Panel/Q&A - SLCC 2016|website=[[YouTube]]|date=2 September 2016|access-date=12 January 2017}}</ref> anak ketiga dari empat bersaudara dari orang tua asal Inggris, Kelly dan Robert Brown.<ref>{{cite web|url=https://www.hollywoodreporter.com/news/stranger-things-stars-father-demands-935013|title='Stranger Things' Star's Father Demands Cash From Agents to Represent Her (Exclusive)|website=[[The Hollywood Reporter]]|first1=Borys|last1=Kit|first2=Matthew|last2=Belloni|date=4 October 2016|access-date=4 December 2017}}</ref> Keluarga tersebut pindah ke [[Bournemouth|Bournemouth, Dorset]] saat Brown berusia sekitar empat tahun dan kemudian ke [[Orlando, Florida]] empat tahun kemudian.<ref>{{cite web|url=https://nypost.com/2017/09/15/at-13-stranger-things-star-millie-bobby-brown-is-an-icon-in-the-making/|title=At 13, 'Stranger Things' star Millie Bobby Brown is an icon in the making|website=[[New York Post]]|first=Gregory E.|last=Miller|date=15 September 2017|access-date=4 December 2017}}</ref>
Brown tinggal di [[London]] dan [[Atlanta|Atlanta, Georgia]].<ref>{{cite web|url=https://variety.com/2017/tv/features/millie-bobby-brown-stranger-things-season-2-eleven-1202602487/|title=How 'Stranger Things' Star Millie Bobby Brown Made Eleven 'Iconic' and Catapulted Into Pop Culture|website=[[Variety (magazine)|Variety]]|first=Debra|last=Birnbaum|date=31 October 2017|access-date=4 December 2017}}</ref>
