Horsfield was raised in Eccles, Lancashire and attended Eccles College before studying English Language and Literature at Newcastle University.
Horsfield initially worked at The Royal Shakespeare Company for two years. She produced her first plays at the Theatre Royal Stratford East and at the RSC Festival at the Barbican.
In 1983, Horsfield won the Television Playwrights Award and made the Resident Writer at The Liverpool Playhouse.
Horsfield's first television adaptation was an episode for Crown Court. Since then, she has wrote for shows such as All the Small Things and Sex, Chips and Rock 'n' Roll.
In 2014, Horsfield was approached by Karen Thrussell and Damien Timmer at Mammoth Screen and asked to consider adapting the first two Poldark novels. She agreed and read the novels, and has since wrote for every series to date.
Horsfield is married to Martin Wenner.