Edmundson was born in Liverpool, in 1964. Most of her childhood was spent in northern England, on the Wirral and in Chester. Edmundson studied drama at Manchester University. After her studies, Edmundson acted with Red Stockings, a female agit-prop company, for whom she wrote the musical comedy Ladies in the Lift in 1988. This was her first solo attempt at writing for the stage. After leaving Red Stockings, she acted throughout northwest England.
Edmundson's first play Flying was produced at the National Theatre Studio in 1990. In 1992, her adaptation of Leo Tolstoy's Anna Karenina, produced by Shared Experience, won a Time Out Award and a TMA Award; the production toured nationally and internationally. In 1993, Edmundson's original play The Clearing, which won the John Whiting Award, was staged at the Bush Theatre. In 1994, her adaptation of George Eliot's The Mill on the Floss was also produced by Shared Experience, again touring nationally and internationally; Edmundson won a Time Out Award for The Clearing and The Mill on the Floss. In 1996, Shared Experience staged her adaptation of Tolstoy's War and Peace at the National Theatre in a production co-directed by Nancy Meckler and Polly Teale and starring BAFTA Award-nominee Anne-Marie Duff; the play was nominated for a Writers' Guild Award for Best Play.
In 2002, Edmundson's play Mother Teresa is Dead was produced at the Royal Court Theatre. In 2004, her adaptation of Mary Webb's Gone to Earth was produced by Shared Experience at the Lyric Hammersmith and on tour; it was nominated for a TMA Award. Edmundson's adaptation of Jamila Gavin's novel Coram Boy premiered at the National Theatre in November 2005, directed by Melly Still and starring Olivier Award-winner Bertie Carvel and Tony Award-nominee Paul Ritter; Edmundson received a Time Out Award and was nominated for an Olivier Award. The play came back for a revival at the same venue a year later, again directed by Still and starring Carvel. Her adaptation of Euripides' Orestes, toured in the UK and played at the Tricycle Theatre with Shared Experience in 2006. Coram Boy was revived at the Imperial Theatre on Broadway in 2007, starring Emmy Award-winner Uzo Aduba and Tony Award-nominee Jan Maxwell, receiving six Tony Award nominations. In 2008, Edmundson amended her adaptation of War and Peace, turning it into a two-part play; this production was staged on tour by Shared Experience. In the same year, her musical adaptation of Isabel Allende's Zorro was produced at the Garrick Theatre, directed by Christopher Renshaw and starring Olivier Award-winner Lesli Margherita and Olivier Award-nominee Emma Williams; Edmundson was nominated for an Olivier Award for Best New Musical. In 2009, Edmundson's adaptation of Pedro Calderón de la Barca's Life Is a Dream was produced at the Donmar Warehouse, starring BAFTA Award-winner Dominic West.
In 2010, Edmundson's musical adaptation of Arthur Ransome's novel Swallows and Amazons was first produced at the Bristol Old Vic, directed by Tom Morris. The next year, the show transferred to the Vaudeville Theatre; the play was nominated for an Evening Standard Theatre Award. Edmundson took part in the Bush Theatre's 2011 project Sixty-Six Books, for which artists wrote a piece based upon a book of the King James Bible; Edmundson wrote a piece entitled In the night, a promise, based on Zephaniah. The same year, her adaptation of Coram Boy was revived at the Bristol Old Vic, again directed by Melly Still. In 2012, her play about Juana Inés de la Cruz, The Heresy of Love, was produced by the Royal Shakespeare Company at the Swan Theatre in Stratford-upon-Avon, directed by Nancy Meckler. The same year, Edmundson's adaptation of Swallows and Amazons was revived for a national tour. Also in 2012, Edmundson's play Mary Shelley was produced on a nationwide tour, including the Tricycle Theatre and the Liverpool Playhouse, by Shared Experience, directed by Polly Teale. In 2014, Edmundson's adaptation of Émile Zola's novel Thérèse Raquin was produced at the Theatre Royal, Bath, starring Olivier Award-winners Alison Steadman and Desmond Barrit. In 2015, The Heresy of Love was revived for a run at Shakespeare's Globe. Edmundson's adaptation of Thérèse Raquin was produced by Roundabout Theatre Company at Studio 54 on Broadway from 2015 to 2016, starring Academy Award-nominee Keira Knightley and Tony Award-winner Judith Light; the play was nominated for Outstanding New Broadway Play at the 2016 Outer Critics Circle Awards. Simultaneously, the RSC premiered her play Queen Anne in Stratford, directed by Natalie Abrahami. From June to September 2017, the RSC is producing Queen Anne at the Theatre Royal Haymarket, again directed by Abrahami and starring Golden Globe-winner Romola Garai.
Edmundson has written two short films for television: One Day, broadcast on BBC Two in July 1991, and Stella for Channel 4. In 2015, she wrote two episodes of ITV drama The Suspicions of Mr Whicher, Beyond the Pale and The Ties that Bind, starring BAFTA Award-winners Paddy Considine and Tim Pigott-Smith. In September of the same year, Edmundson's feature-length adaptation of J. B. Priestley's An Inspector Calls, starring BAFTA Award-winners David Thewlis and Miranda Richardson, was broadcast on BBC One; the adaptation won the 2016 Broadcasting Press Guild Award for Best Single Drama and was nominated for two 2016 British Academy Television Craft Awards. Recently, Edmundson wrote the upcoming film Mary Magdalene, directed by Emmy Award-nominee Garth Davis and starring Academy Award-nominees Rooney Mara and Joaquin Phoenix.
Edmundson has adapted numerous literary classics for BBC Radio 4, including Virginia Woolf's The Voyage Out in 2006, Thomas Hardy's The Mayor of Casterbridge in 2008 and Jane Austen's Sense and Sensibility in 2013.
Edmundson has been a Fellow of the Royal Society of Literature since 2015.1992 Time Out Award for Anna Karenina
1992 TMA Award for Anna Karenina
1993 John Whiting Award winner for The Clearing
1994 TMA Award for The Clearing and The Mill on the Floss
Nomination: 1996 Writers' Guild Award for Best Play for War and Peace
Nomination: 2004 TMA Award for Best Touring Production for Gone to Earth
2005 Time Out Award for Coram Boy
Nomination: 2006 Laurence Olivier Award for Best New Play for Coram Boy
Nomination: 2009 Laurence Olivier Award for Best New Musical for Zorro
Nomination: 2012 Evening Standard Theatre Awards' Ned Sherrin Award for Best Musical for Swallows and Amazons
2015 Windham–Campbell Literature Prize (Drama)