King began her career in guest-starring roles on television, and starred in the short-lived CW medical comedy-drama series Emily Owens, M.D.

After her breakthrough as Michaela Pratt in the ABC legal drama series How to Get Away with Murder (2014–2020), she received praise for portraying Cherry Turner in the historical film The Birth of a Nation (2016).

For her performance in the Apple TV+ miniseries Lessons in Chemistry (2023), King was nominated for the Primetime Emmy Award for Outstanding Supporting Actress in a Limited or Anthology Series or Movie.

[4][5] At Yale University, King performed in a number of productions, including A Midsummer Night's Dream, Little Shop of Horrors, and Angels in America: A Gay Fantasia on National Themes.

She made her television debut in 2010, as a guest star in the CBS police procedural Blue Bloods, and later appeared on Person of Interest, The Blacklist and Deadbeat.

[12] In early 2014, King had a recurring role as Ali Henslee in the ABC medical drama series Black Box, starring Kelly Reilly.

King plays the role of Michaela Pratt, one of the five lead students, alongside Alfred Enoch, Jack Falahee, Matt McGorry, and Karla Souza.

[16][17] Also in 2015, she was cast as the female lead in the historical drama film The Birth of a Nation, based on the story of the 1831 slave rebellion led by Nat Turner.

Variety placed her in their list of one of the "Biggest Breakthrough Performances" at Sundance, writing that "King transforms herself from contemporary glamour girl to 19th-century slave in Nate Parker's festival smash.

A harrowing scene late in the film opposite Parker as Nat Turner reveals a depth and range King has never been asked to deliver in her small screen work, and a potential new star is born.

[38][39] During her career, King has graced the covers of numerous magazines, including Vanity Fair,[40] Elle, Marie Claire, Nylon,[41] Glamour, Essence,[42] Entertainment Weekly[43] and Shape.