As a writer-director, Fennell is known as the showrunner for season two of the BBC spy thriller series Killing Eve (2019), which earned her two Primetime Emmy Award nominations.

She made her feature film directorial debut with the thriller Promising Young Woman (2020), for which she won the Academy Award for Best Original Screenplay, and received nominations for Best Picture and Best Director.

Fennell also wrote the book for Andrew Lloyd Webber's musical Cinderella (2021) and directed her second film, the psychological thriller Saltburn (2023).

Yossman, "part of a rarefied...social set whose family names I recognized from gossip columns and history books… Balfour, Frost, von Bismarck, Guinness, Shaffer.

Titled Chukka, it was a romantic comedy about a group of teenagers who fight the closure of their school by taking on the rich kids at polo.

Fennell starred as Agnes in the Channel 4 sitcom Chickens (2011–2013), alongside Simon Bird, Joe Thomas and Jonny Sweet.

[20] On 23 October 2018, it was announced that Fennell would play Camilla Shand in the third season of the Netflix period drama series The Crown.

She continued playing the role in the fourth season, which earned her a nomination for the Primetime Emmy Award for Outstanding Supporting Actress in a Drama Series.

[21] In January 2019, it was announced Fennell would produce, write and direct the comedy thriller film Promising Young Woman, starring Carey Mulligan.

[24] The movie premiered at the 2020 Sundance Film Festival to critical acclaim[25] with, as of August 2023, a 90% score on Rotten Tomatoes, and a critical consensus of: "A boldly provocative, timely thriller, Promising Young Woman is an auspicious feature debut for writer-director Emerald Fennell -- and a career highlight for Carey Mulligan.

[26] In January 2020, Andrew Lloyd Webber announced he would collaborate with Fennell on the musical Cinderella, which opened in London in August 2021.

[30] It made the transfer to Broadway in 2023, entitled Bad Cinderella, with Fennell's original book adapted by playwright Alexis Scheer.

[35] In July 2022, Cuban-Spanish actress Ana de Armas stated in an interview with Elle that Fennell was hired to contribute to the script of the John Wick spin-off film Ballerina as one of its writers.

Also in 2023, Fennell wrote, produced, and directed her second feature film, Saltburn, a black comedy and psychological thriller starring Barry Keoghan, Jacob Elordi, Rosamund Pike and Richard E.

On Rotten Tomatoes, the film's critical consensus states: "Emerald Fennell's candy-coated and incisive Saltburn is a debauched jolt to the senses that will be invigorating for most.