Atwell's made her professional debut in 2005, portraying Io, a maiden exiled by Zeus, in "Prometheus Bound" at the Sound Theatre in London. The "British Theatre Guide" praised her performance, writing that Atwell "makes us respond to the anguish without for a moment inviting a chuckle at her bandaged hands". The following year, she starred as the protagonist's wife, Bianca, in "Women Beware Women" at the Royal Shakespeare Company. "The Guardian" praised Atwell for projecting "the right seductive beauty". Atwell then appeared in two productions at the Royal National Theatre, both directed by Nicholas Hytner. She portrayed Belinda, a SoHo PR worker and one of three love interests, in "Man of Mode". Atwell also starred in "Major Barbara" for which she received an Ian Charleson Commendation. In 2009, Atwell made her West End debut as Catherine, the adopted niece in a troubled household, in Lindsay Posner's "A View From the Bridge". "Variety" praised her for having an "ideal freshness" and girlishness while still able to shift into uncontrolled rage; her performance was later nominated for an Olivier Award.