The daughter of a British Army officer, Lt. Colonel Johnny Fielding and Sheila Fielding, she was raised Catholic and spent much of her childhood in Malaysia and Nigeria in 1979, and a period in Malvern above her grandparents' betting shop. While studying at the Berkhamsted Collegiate boarding school, she won a place at Robinson College, Cambridge to study law, but abandoned it and spent a gap year which included five months in a West Bank kibbutz picking watermelons, and as an usherette at the Oxford Apollo; before embarking on the study of acting at the Royal Scottish Academy of Music and Drama.
After graduation she worked for the Royal National Theatre and the Royal Shakespeare Company, coming to the attention of critics in 1993's RSC production of Tom Stoppard's Arcadia, in which she created the role of Thomasina, and then most notably in John Ford's The Broken Heart for which she won the Dame Peggy Ashcroft Award for Best Actress. Also in 1993, she was Agnes in The School for Wives at the Almeida Theatre, for which she won the Ian Charleson Award. She made her Broadway theatre debut in 2003 in Noël Coward's Private Lives. She has also appeared in numerous radio plays for the BBC, including playing Esme in Tom Stoppard's Rock 'n' Roll, a role she also played in the West End. More recently, she appeared in the BBC TV mini-series Cranford.
In 2009 she appeared as Daisy alongside Timothy West in the BBC Radio 4 adaptation of John Mortimer's "Rumpole and the Penge Bungalow Murders". She has also appeared in the crime drama 'Death in Paradise' playing the part of Astrid Knight. (Season 1 Episode 4). In 2014, she appeared in another crime drama DCI Banks (Series 3 Episodes 17 & 18).
In 2018, Fielding appeared in EastEnders as Ted Murray's (Christopher Timothy) daughter.
In November 2018, she provided the voice for the alien Kisar in the Doctor Who episode "Demons of the Punjab".