Horsfield has worked for the Gulbenkian Studio, Newcastle, and for the Royal Shakespeare Company. Her first plays were Out on the Flool, Away from it All and All You Deserve. Horsfield was commissioned to write Red Devils (which later became a trilogy) by the Liverpool Playhouse Studio in 1983, where she stayed for a year under the Thames Television Playwright Scheme. The follow-ons from Red Devils were True Dare Kiss, later to be adapted for a TV series, and Command or Promise. A further play was titled Touch and Go (later renamed Revelations).

In the 1980s Horsfield began her television career by writing 3 series of Making Out and an episode of Crown Court. In the 1990s Horsfield wrote the series Sex, Chips & Rock n' Roll, Born to Run, and The Riff Raff Element. In the 2000s she began to produce her shows as well as write them; 4 series of the hair salon based Cutting It and the mystery drama True Dare Kiss are examples of her work for BBC One.

Horsfield has twice won BAFTA Best Drama Series award nominations, for The Riff Raff Element in 1994 and for Cutting It in 2003.

She has recently written a show called All The Small Things for BBC One, which follows the trials and tribulations of a northern church choir.