She is known for her roles as Mary Hawkins in the Starz series Outlander[1] and Sarah Jessica Parker's character's daughter in the comedy film All Roads Lead to Rome.

To prepare for the part, Day conducted intensive "research into the experience of women who [had] been in that situation"[14] and learned sign language to portray the deaf character.

[17] In 2014, while working with Scottish actor Sam Heughan on Heart of Lightness, a film based upon Henrik Ibsen's play The Lady of the Sea, the Outlander star told her about the character and suggested Day for the part.

[19] Later that year she starred alongside Uma Thurman in Summit Entertainment's Down a Dark Hall, an American-Spanish horror film, based on the novel by Lois Duncan.

In 2021 she starred alongside Russell Tovey and Sian Clifford in the comedy Real Love, and voiced the character Meteion in Final Fantasy XIV: Endwalker.