She appeared as an isolated popular girl in The Blackcoat's Daughter (2015) and starred as a bold aspiring model in Sing Street (2016).

Boynton gained recognition for starring as Mary Austin in the biopic Bohemian Rhapsody (2018), which received numerous accolades and was a box office success.

[22] In 2013, she starred in Saint Raymond's music video for the lead single from his debut extended play, "Fall At Your Feet".

[30][31] Boynton appeared as "sullen mean girl" Rose alongside Emma Roberts and Kiernan Shipka in The Blackcoat's Daughter (2015).

[34] Boynton starred in the coming-of-age film Sing Street (2016) as Raphina, an aspiring model with "huge hair, bright make-up and colourful clothes".

[41][42] Noel Murray of the Los Angeles Times praised Sackhoff and Boynton's "volatile chemistry", stating that "they bring so much life to the material that it’s almost like they’ve been tricked into thinking they’re in a better movie.

[45][46][47][48] In the same year, Boynton starred as Allison Adams, a college student turned drug addict, in the Netflix series Gypsy (2017).

[51][52][53] Boynton starred opposite Rami Malek as Freddie Mercury's partner Mary Austin in Bohemian Rhapsody (2018).

[56][57] Time praised her for playing the role with "charm and vigour" while the Irish Independent wrote that her scenes were the "quietest, most touching moments" of the film.

[62] Since 2019, Boynton has starred in the Netflix series The Politician, portraying Astrid Sloane, the protagonist's "ruthless" and "hyperbolically privileged" rival.

[65][66] The Hollywood Reporter praised Boynton's "razor-sharp delivery" but remarked that her written dialogue prevented her from being "spectacular".

[70] Boynton had a supporting role as Charlotte, a Harrods shop assistant, in Locked Down (2021), released on HBO Max.

[73][74] In 2022, Boynton appeared in ITV's six-part miniseries The Ipcress File as Jean Courtney, Harry Palmer's colleague, and also starred in the Stephen Williams-directed biographical film Chevalier as Marie Antoinette.

[78] In August 2022, it was announced that Boynton would star in The Greatest Hits, written and directed by Ned Benson for Searchlight Pictures;[70] the film was released in April 2024 to mixed reviews.

[83][84] Boynton divides her time between London, New York, and Los Angeles;[10][85] she considers herself to be "painfully British"[3][85] while simultaneously feeling "sort of" American.