Kirby began her film career with smaller roles in various British films. These early experiences provided valuable on-set experience and helped her develop her craft.

Starred in the crime-drama "The Rise".

Appeared in Richard Curtis's romantic comedy "About Time".

Starred in "Queen and Country" directed by John Boorman.

Had a minor role in the Wachowskis' science fiction film "Jupiter Ascending".

Starred in the adventure film "Everest".

Appeared in the drama "Genius", as Zelda Fitzgerald.

Kirby gained international recognition for her role as Alanna Mitsopolis/White Widow in "Mission: Impossible – Fallout". This action film showcased her versatility and expanded her fanbase.

Starred in "Hobbs & Shaw", a spin-off from the Fast & Furious franchise.

Starred in "The World to Come" directed by Mona Fastvold.

Kirby received widespread critical acclaim for her performance in "Pieces of a Woman", a powerful drama exploring grief and loss. Her portrayal of Martha Weiss earned her numerous awards and nominations, including an Academy Award nomination for Best Actress.

Kirby played Empress Joséphine in Ridley Scott's historical epic "Napoleon", starring alongside Joaquin Phoenix. Her performance received positive reviews.

She reprised her role of Alanna Mitsopolis in "Mission: Impossible – Dead Reckoning Part One".

Kirby continues to be involved in a variety of film projects, showcasing her commitment to diverse and challenging roles.