Her first major part was in the play Fireface at the Royal Court Theatre, which she took just before graduation. In 2001 she won the Critics' Circle Theatre Award for Best Newcomer in 2001 for her performances in Redundant at the Royal Court Theatre and Boston Marriage at the Donmar Warehouse. In 2003, she won the TMA Theatre Award for Best Supporting Actress in A Midsummer Night's Dream. Since then she has performed leading roles in fourteen theatre productions, most recently alongside James McAvoy in the 2009 West End production Three Days of Rain. In 2011 she appeared in Greenland at the National Theatre.
Marshal starred as Cleopatra in HBO's Rome. Since 2009 she has played Lady Sarah Hill in the three series of the BBC One drama Garrow's Law and, with Julie Walters, in the BBC One Drama A Short Stay in Switzerland. She starred also in ITV's new series of Agatha Christie's Marple.
In January 2013 Marshal played Eileen Blair, wife of Eric Blair, in the BBC Radio 4 drama The Real George Orwell – Homage to Catalonia.
