Langford first appeared in several small independent films, including Story of Miss Oxygen (2015), Imperfect Quadrant (2016), and Daughter (2016). She portrayed the lead character in Daughter, which debuted at the 2016 Cannes Film Festival. In 2016, after declining the offer from Western Australian Academy of Performing Arts, Langford auditioned for Will, a television series centered on the young life of William Shakespeare. She did not get the role, which was instead given to Olivia DeJonge.
Langford auditioned for the role of American high school student Hannah Baker in the mystery teen drama TV series 13 Reasons Why, over Skype. She had only 10 days to get an O-1 visa as she had not worked in the United States before. She has received critical acclaim for her work on the show. Langford researched the role, speaking with a representative of the sexual assault awareness campaign "It's On Us" and a psychiatrist who specializes in adolescence. In December 2016, she signed with the William Morris Endeavor agency. Langford reprised the role in the second season of the series, released on May 18, 2018. On May 25, 2018, Langford confirmed that she will not return as Hannah Baker in the third season of the series.
Langford appeared in her first feature film, The Misguided, an independent comedy-drama by Shannon Alexander, which premiered in January 2018. She also starred as Leah in the 2018 film Love, Simon, an adaptation of the coming-of-age novel Simon vs. the Homo Sapiens Agenda, by Becky Albertalli.
On September 12, 2018, it was announced that Langford had been cast in the upcoming web television series Cursed. Set in an re-imagined Arthurian world, Langford will portray Nimue, a teenage girl destined to become the Lady of the Lake. The series is based on the novel of the same name by Frank Miller and Tom Wheeler and is set to premiere on Netflix in 2019.
In October 2018, it was announced she had been cast in Avengers: Endgame.
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