Williams wrote a series of Funny or Die sketches, in which she starred as newlywed Kate Middleton, with English actor and model Oliver Jackson-Cohen as Prince William, Duke of Cambridge. Williams appeared as Danielle in an episode of the third season of The League titled "The Guest Bong". Williams has appeared as a recurring character, Cheryl, in the CollegeHumor series Jake and Amir.
On December 4, 2014 she starred in the title role on NBC's live television presentation of the musical Peter Pan Live!, opposite Christopher Walken as Captain Hook. In November 2016, Williams appeared in Past Forward, a short film collaboration between David O. Russell and Prada.
In 2017, Williams starred in the hit horror film Get Out, directed by Jordan Peele, who became convinced she was right for the role of Rose after seeing her performance in Peter Pan Live! In the same year, Williams was cast in the role of Charlotte on the Netflix horror film, The Perfection. The film was released on May 24, 2019. In 2018, Williams was cast in a recurring role as Kit Snicket throughout the second and third seasons of the Netflix comedy drama series A Series of Unfortunate Events.