Fox was born in London, England. Her mother is actress Joanna David (née Joanne Elizabeth Hacking) and her father is actor Edward Fox. She was educated at Bryanston School and the University of Oxford..

She had a relationship with comedian Vic Reeves and in July 2005 married British actor Jared Harris, the son of the late Irish actor Richard Harris. She speaks German and French, she is a keen kickboxer. She has a brother Freddie and a half sister Lucy.

Fox first appeared as the sister of Colin Firth's Mr. Darcy in the 1995 television adaptation of Pride and Prejudice, followed by her role as the second Mrs. de Winter in the 1997 TV adaptation of Rebecca, opposite Charles Dance.

She played Jeannie Hurst in the 2000 remake of Randall and Hopkirk (Deceased). In 2003, she played Jane Seymour in a two-part television biopic on King Henry VIII. She also played the title role in Katherine Howard at the Chichester Festival Theatre, Chichester, England in 1998.

In 2004, she played Lady Margaret in Part 2 of Gunpowder, Treason and Plot, the mini-series about James I of England and the Gunpowder Plot. In 2005 she played Rosie Jones in the film Keeping Mum and in 2008 she played Sister Jean in Baillie Walsh's Flashbacks of a Fool with Daniel Craig.

She was cast as Lynne Frederick in the 2004 film The Life and Death of Peter Sellers starring Geoffrey Rush in the lead role. A whole section of the film focusing on the Frederick/Sellers relationship was removed in the final edit. The deleted scenes with Fox can be found with others among the special features on the DVD release of the film.