Born and raised in London, Atwell studied acting at the Guildhall School of Music and Drama, and made her stage debut in the 2005 play Prometheus Bound. She appeared in multiple West End productions and on television, gaining recognition for her breakthrough role as Lady Elizabeth Foster in The Duchess (2008), for which she was nominated for a British Independent Film Award for Best Supporting Actress. Her leading performance in the miniseries The Pillars of the Earth (2010) earned her a nomination for the Golden Globe Award for Best Actress - Miniseries or Television Film.
Atwell rose to prominence for her portrayal of Peggy Carter in the superhero film Captain America: The First Avenger (2011), a role she would reprise in the action-adventure series Agent Carter (2015–2016) and the Marvel Cinematic Universe films Captain America: The Winter Soldier (2014), Avengers: Age of Ultron (2015), Ant-Man (2015), and Avengers: Endgame (2019). She also had roles in the war drama Testament of Youth (2014), the romantic fantasy Cinderella (2015), and the fantasy comedy-drama Christopher Robin (2018).
On the stage, Atwell achieved a Laurence Olivier Award nomination for Best Supporting Actress in A View from the Bridge (2010) and nominations for Best Actress for her leading performances in The Pride (2013) and Rosmersholm (2020). Atwell began hosting the podcast series True Spies in 2020, which gives an insight into the world of espionage, asking listeners what they would do in real life spy situations.