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 playing when she was thirteen years old. In preparation for her role as Beth March in the 2019 film adaptation of Little Women, she started practising the piano again.
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 March in Greta Gerwig's adaptation of Louisa May Alcott's novel Little Women, co-starring alongside Saoirse Ronan, Emma Watson, Florence Pugh, Laura Dern, Timothée Chalamet, and Meryl Streep.
