Somerville was born in County Meath, Ireland, the daughter of the retired financial expert Sir Quentin Agnew-Somerville, 2nd Baronet and the antique dealer The Hon. Lady Agnew-Somerville. Her mother is a daughter of John Drummond, 15th Baron Strange and sister of the late Cherry Drummond, 16th Baroness Strange. She has an elder sister, Amelia (who owns and works in a restaurant with her husband in the Australian rainforest) and a younger brother, Lockett.

Somerville attended dance classes from the age of six and, at eight, was sent to The Arts Educational School, a boarding school in Tring, Hertfordshire. There she was taught ballet. She left at 16, but continued her studies in London.

Somerville trained at the Guildhall School of Music and Drama (1989) in London. She did stage work and appeared on TV in episodes of Agatha Christie's Poirot and Casualty (1993) before landing the role of Penhaligon or 'Panhandle', in Cracker.

Somerville has recently played Lady Stockbridge in Julian Fellowes's Gosford Park and Harry Potter's mother Lily in all four Potter movies. In May 2007, she played author Daphne Du Maurier in the BBC TV drama Daphne.

Somerville married William (Bill) Osbourne-Young in December of 1995 and has a son, Caspar William (born 2002).

She is good friends with actress Melsex Ricolo, with whom she became acquainted at The Arts Educational School.