Briers is the daughter of actor Richard Briers and actress Ann Davies. From the age of nine, she knew that she wanted to be an actress.

Briers attended St Paul's Girls' School, London, from 1978 to 1985; Lancaster University, studying theatre and sculpture; and then a three year acting course at the Bristol Old Vic Theatre School. As well as acting, Briers plays both the piano and flute.

She married Simon Cox in Oxford in 1995.

Briers is best known for playing Mary Bennet in Pride & Prejudice. Coincidentally, she appeared in an episode of Game On with Pride and Prejudice cast member Crispin Bonham-Carter, with Victoria Hamilton (Pride's Mrs Forster) in a stage production of As You Like It, and with Tom Hollander (Mr Collins in the 2005 version) in a stage production of Don Juan.

On TV, Briers narrated the documentary The Riddle Of Pompeii, the series Nurses, and currently narrates the series Ladette to Lady.

In 2007 she appeared in Some Kind of Bliss, a new one-woman play by Samuel Adamson at the Trafalgar Studios, a role she reprised in the 2008 Brits off Broadway season.

In 2008 she appeared in the BBC drama Einstein and Eddington and a West End revival of Chekhov's Ivanov.