She first became known for starring as Marnie Michaels in the HBO comedy-drama series Girls (2012–2017), which earned her a Critics' Choice Award nomination.

Her latest on-screen appearance was for the role of Lucy Smith in Showtime's Peabody Award-winning miniseries Fellow Travelers (2023).

[1][2][3][4] She is the daughter of former NBC Nightly News anchor and managing editor Brian Williams[5] and TV producer Jane Gillan Stoddard.

[10] She was also a member of the improv comedy troupe Just Add Water for four years and acted in the YouTube series College Musical.

[5][15] In 2010, Williams performed a mashup of "Nature Boy" set to RJD2's "A Beautiful Mine", the theme song for the television series Mad Men.

"[33] To prepare for the role, Williams isolated herself from the cast and crew during production in order to understand the psychopathic nature of her character.

[35] Johnny Hoffman of MovieWeb praised her for being "suspicious and charming from beginning to end" and wrote that "Williams proved, with this character, that she is perfectly capable of portraying an effective villain.

[37][38] Williams was cast in the role of Charlotte on the Netflix horror film The Perfection,[39] which was released on May 24, 2019, to positive reviews,[40] and her performance was called "compelling".

[49][50] Jason Zinoman of The New York Times said that she had a "knack for playing it straight" and further wrote that she "capably updates the mad-scientist archetype, refusing to pause and ask questions while inventing a doll of the future, one who pairs with a child and adjusts to their needs, filling in as best friend and big sister.

[52] Kristy Puchko of Mashable said that she "solidly grounds the human drama within this scary sci-fi premise of a killer doll.

[66] In late 2019, Williams began dating German actor Alexander Dreymon, whom she met while filming Horizon Line.