Atwell was born in London, the only child of Allison, a motivational speaker, and Grant Atwell, a landscape photographer and shaman. Atwell's mother is English and her father is an American from Kansas City, Missouri and of part Native American descent (Atwell has dual citizenship between the U.S. and the U.K.). Her parents separated when she was two.
Atwell took A-levels at the London Oratory School before training at the Guildhall School of Music and Drama graduating in 2005, in place of studying philosophy and theology at King's College London.
Career Atwell debuted in a 2005 Pringles TV commercial and served on the jury for the 2007 British Independent Film Awards.
Her first feature film role was in Woody Allen's 2007 film Cassandra's Dream, where she acted opposite Ewan McGregor and Colin Farrell. In 2008 she appeared in the films The Duchess (as Lady Bess Foster) and Brideshead Revisited (as Lady Julia Flyte).
AMC Television and ITV have announced that she will appear in a 2009 TV miniseries remake of The Prisoner, as a character code-named No. 41-5. In January of the same year she will make her West End debut in Lindsay Posner's revival of A View From The Bridge.