He grew up on the East Kent coast, primarily in Deal, within a family fractious with "parental separation, secret illegitimacy, alcoholism and genteel poverty".
In his eighteenth year, he attended Bristol University to study geography, after which he had any number of jobs—fundraising and teaching, among others, as well as editing for the Daily Mail.
In 1978, at age 34, he retired from the newspaper in order to pursue novel-writing as his primary career, inspired by some long-ago reading of Frances Hodgson Burnett's The Secret Garden.
It was re-published in 2008 as City of Dark Hearts with some significant revisions and cuts under the pen name James Conan.
After almost fifteen years, Horwood returned to his hallmark genre of fantasy, publishing the first novel in his Hyddenworld quartet in 2010.