When five years old, she became passionate about acting after being taken to see ballets by her father, and joined Stagecoach Theatre Arts at the same age.

[9][10] In 2014, Day appeared in a YouTube prank video where she masquerades as the girlfriend of Caspar Lee, and flirts with his roommate Joe Sugg.

[8] In the same year, she appeared in the music videos for Lea Michele's "On My Way", Arctic Monkeys' "Arabella" and 5 Seconds of Summer's "She Looks So Perfect",[11] and in the seventh episode of the fourth series of Sky One's Trollied.

[12] In 2015, Day appeared as Angelica in the television film Casanova,[11] and played Olalla Jekyll/Hyde in the ITV series Jekyll and Hyde.

[5] In 2016, Day appeared in the Instagram original series Shield 5 as Amy Williams,[14] and joined The Royals in the recurring role of Angie which she played till 2018.

[19] In March 2021, Day was cast to recur in the second season of the CW series Batwoman to take over the role of Kate Kane from Ruby Rose.

[8] She is also a trained swimmer, and began preparing to participate in the 2012 Summer Olympics, but decided to leave swimming to focus on her acting career.