Walker's first school was Silverdale Preparatory West Acton, London.

Walker began her career as a dancer, but had to abandon dancing after an injury at the age of 18. She moved from London Drama Center to the Royal Shakespeare Company, where she played bit parts for six months before graduating to small roles on television.

Walker landed the title role in the television series Lorna Doone before making her feature debut in Shogun Mayeda (1991). In that same year she appeared in two other films, Les Equilibristes and Mike Newell's Enchanted April, in which she played an aristocrat eager to escape the attentions of her persistent male admirers. She first garnered international attention as a single-minded member of an Irish terrorist splinter group in Phillip Noyce's Patriot Games. (Though the organisation is Irish, Walker's character is English which has some plot significance.)

In late 2005, she appeared in the first season of the HBO/BBC2 original television series Rome, as Atia of the Julii — a role that earned her a Golden Globe nomination for Best Performance by an Actress in a Television Series - Drama. She reprised the role in the second season of the series that began airing in January 2007.

Her future projects in 2007 include CBS television drama, Cane, which is scheduled to air in the fall.