Ramsey portrayed Lyanna Mormont in the HBO fantasy drama series Game of Thrones, starting from season 6. Her character was originally set to appear in just one scene but became a recurring role due to her impressive performance.

Ramsey played the main role of Mildred Hubble in the CBBC series The Worst Witch. She won the Young Performer award at the 2019 British Academy Children's Awards for this role.

Ramsey voiced the title character in the Netflix original series Hilda, for which she won the 2019 BAFTA award for best 'Children's Animation'.

Ramsey was cast as Ellie in the HBO adaptation of the video game The Last of Us, alongside Pedro Pascal. This role earned her a nomination for Outstanding Lead Actress in a Drama Series at the 75th Primetime Emmy Awards.