Scanlen was born in Sydney, New South Wales, Australia, and has a fraternal twin sister named Annabel. She learned the piano when she was about 7 years old, but stopped when she was 13 years old. While in high school, Scanlen was cast in the recurring role of Tabitha Ford on television soap opera Home and Away. Scanlen starred as the titular character in the 2018 short film Grace. She then achieved recognition for her portrayal of Amma Crellin in the HBO psychological thriller miniseries Sharp Objects, in which she stars alongside Amy Adams. She made her professional theatre debut in Sydney Theatre Company`s 2019 production of Lord of the Flies, directed by Kip Williams. She played the role of Eric for the play`s run. She made her feature film debut as Milla Finlay in Shannon Murphy`s Babyteeth which premiered in competition at Venice Film Festival. In 2019, she portrayed Beth in Greta Gerwig`s adaptation of Louisa May Alcott`s novel Little Women alongside Saoirse Ronan, Emma Watson and Florence Pugh.