Mortimer was born in London, England, the daughter of Sir John Mortimer QC (lawyer and dramatist famous for Rumpole of the Bailey) and Penelope (née Gollop). Her maternal grandfather was a pig farmer. She has a younger sister, Rosie, and a half brother, Ross Bentley. Mortimer studied at St Paul's Girls' School, where she appeared in several student productions. After St. Paul's, she moved on Read more...

Mortimer performed in several plays while studying at Oxford University, and while acting in a student production she was spotted by a producer who later cast her in a supporting role in a television adaptation of Dame Catherine Cookson's The Glass Virgin (1995). Subsequent television roles included Sharpe's Sword. Her first film role was opposite Val Kilmer in 1996's The Ghost and the Darkness. Read more...

In 2000, Mortimer met American actor Alessandro Nivola, while both were starring in Love's Labour's Lost. The couple married on 3 January 2003. A Mexican punk band performed at their wedding. Mortimer gave birth to the couple's son Samuel on 23 September 2003.