[6] Her theatrical career includes a performance as the Queen in Shakespeare's Cymbeline opposite Tom Hiddleston, portraying Mag Wildwood in Breakfast at Tiffany's, and "standing out" as Lucifer in Marlowe's Doctor Faustus (2010).

On screen, Christie debuted with a 2007 short film titled The Time Surgeon, written and directed by Nathaniel Mellors.

In 2009 she joined Patrick Wolf in the music video for "Damaris".In July 2011, Christie was cast as the warrior Brienne of Tarth in the second season of HBO's fantasy TV series Game of Thrones.

[17] Her character, an unusually tall, muscular, and plain-looking woman, is a favourite among many readers of the novels, and Christie had been proposed for the role by fans long before auditions took place.

[7] To prepare for the auditions, she started wearing unisex clothing to help her get into her character's more masculine mindset, and took up an intensive training regime, gaining over a stone (6.4 kg) of muscle mass.

[6] According to series co-writer, producer, and author of the novels, George R. R. Martin, she obtained the role practically without debate after an arresting audition at which she appeared already made up and costumed as Brienne.

Nina Shen Rastogi praised her "eloquent and economical physical performance", noting that her walk, stance, and mien effectively conveyed Brienne's single-minded devotion to her self-given quest to become a knight.

[23][24][25][26][27] In addition to her role in Game of Thrones, Christie appeared in the British science fantasy series Wizards vs Aliens by Doctor Who writers Russell T Davies and Phil Ford from 2012 to 2013, in the main role of Lexi, the 17-year-old Princess of the Nekross, the alien species invading earth.

[30] Also in 2015, Christie co-starred in Star Wars: The Force Awakens, as Captain Phasma, one of the film's supporting antagonists.

[33][34][35] In 2017, Christie had a main role in China Girl, the second season of Top of the Lake, as Miranda Hilmarson, a Constable in the Sydney Police Force, and the new partner of the show's main character played by Elisabeth Moss; the character was written especially for Gwendoline by series co-creator Jane Campion, after Christie asked her to be a part of the series.

In January 2024, Christie walked for Maison Margiela during Paris Couture Week, wearing the closing look of the collection.