She has written three novels: Namedropper (1998), Thin Skin (2002), and Cherries in the Snow (2005).

A screenplay by Forrest about the late musician Jeff Buckley, Becoming Music, was bought by Brad Pitt's production company Plan B Entertainment in September 2000. In 2009 her screenplay LIARS (AE) was bought by Scott Rudin at Miramax, with Richard Linklater attached to direct. In 2009 she was listed on Variety's "Top Ten Screenwriters to Watch". Other screenplays in development:Know Your Rights (Film4) and How Could You Do This To Me(Paramount Pictures).

In 2001 Forrest contributed to an anthology on the Writer J.D. Salinger titled Love & Squalor, with an essay describing Salinger's influence on some current young Writers. In 2007 she co-wrote and edited her first non-fiction book, Damage Control – Women on the Therapists, Beauticians, and Trainers Who Navigate Their Bodies, which was an anthology of essays of the emotional pain women suffer for their own physical wellbeing. The book features contributions from many well-known women, including Helen Oyeyemi, Marian Keyes and Sarah Jones.

Forrest was in a relationship with actor Colin Farrell, whom she met in 2008. She refers to him in her memoir as "GH," short for "Gypsy Husband."

Forrest wrote a memoir, Your Voice In My Head (2011), concerning the death of her Psychiatrist and her subsequent break-up with her partner. The memoir was announced as a feature film written by Forrest and directed by Francesca Gregorini with Emma Watson in the lead role.

In June 2012, Forrest married Animal Kingdom actor Ben Mendelsohn They divorced in 2016

Age fifteen Emma Forrest wrote a story on Madonna for the London Evening Standard. She left school to write her "Generation X" column for the Sunday Times writing about various britpop/indie bands "on the road". The vanished Manic Street Preacher member Richey Edwards was the first person interviewed. Forrest has worked for Vogue, Vanity Fair, Harper’s Bazaar, Time Out, The Guardian, NME, Interview and Blackbook, where she interviewed Snoop Dogg and Brad Pitt. Most recently, she contributed an essay to the album Ceremonials by Florence and the Machine.