Horsfield worked at the Gulbenkian Studio, Newcastle (1978–1980), and for Trevor Nunn at The Royal Shakespeare Company (RSC), 1980–82.
Her first plays Out on the Floor and Away from it All were produced at the Theatre Royal Stratford East studio and All You Deserve was performed as part of an RSC Festival at the Barbican.
[1] In 2005, she returned to the theatre to adapt her TV series Sex, Chips & Rock 'n Roll as a musical for the Royal Exchange, Manchester.
In 1982, Horsfield adapted her stage play Out On The Floor for BBC2 and wrote an episode of Crown Court for Granada Television.
Horsfield has twice won BAFTA Best Drama Series award nominations, for The Riff Raff Element in 1994 and for Cutting It in 2003.