Mortimer was born in Finsbury Park, London, England, a daughter of Sir John Mortimer, QC (a lawyer and writer who created Rumpole of the Bailey) and his second wife, Penelope (née Gollop). She has a younger sister, Rosie; two older half-siblings, Sally Silverman and Jeremy, by her father's first marriage, to author Penelope Fletcher; and a half-brother, Ross Bentley, by her father's liaison with actress Wendy Craig. Her maternal grandfather was a pig farmer.
Mortimer studied at St Paul's Girls' School in west London (she was in the year below actress Rachel Weisz), where she appeared in several student productions. She then went on to Oxford University, where she read Russian at Lincoln College and performed in several plays. Before becoming an actress, Mortimer wrote a column for the Daily Telegraph, and was also screenwriter for a screen adaptation of Lorna Sage's memoir, Bad Blood.
Mortimer studied at St Paul's Girls' School in west London (she was in the year below actress Rachel Weisz), where she appeared in several student productions. She then went on to Oxford University, where she read Russian at Lincoln College and performed in several plays. Before becoming an actress, Mortimer wrote a column for the Daily Telegraph, and was also screenwriter for a screen adaptation of Lorna Sage's memoir, Bad Blood.