Born in 1956 in Salford, England, Constantine attended art college and was the manager of several bands. She began writing stories as a child, developing a strong interest in Egyptian, Greek and Roman mythology, as well as the Tarot. In the late 1970s she began developing a fantasy series revolving around a new, hermaphrodite post-human species that would arise to replace humanity. This idea evolved into the Wraeththu Chronicles, a trilogy starting with The Enchantments of Flesh and Spirit (1987).
The series was highly successful, sparking a prolific period lasting into the late 1990s and consisting of the Artemis and Grigori series. However, although she maintained strong sales in the USA, Constantine found sales in the UK dipping and publishers losing interest in her work. In 2003 she founded Immanion Press as a way of keeping her work in print and publishing new work, such as a Wraeththu sequel trilogy. She also published work by other authors, including numerous works related to paganism, myth and mythology. An immense fan of fellow fantasy/horror author Tanith Lee, she republished some of Lee's back catalogue as well. She also founded and edited the Visionary Tongue magazine in the 1990s.
During her career, she published one novella, nine short story collections and twenty-five novels (one in collaboration with Michael Moorcock), as well as collaborating on the Wraeththu roleplaying game.
Her work was notable for breaking new ground in the treatment of gender and sexuality in fantasy fiction, as well as being an author unafraid to embrace fanfiction. Many fans who wrote fiction set in her worlds were encouraged to submit to official anthologies she published, and she encouraged them to seek professional careers in the field. She will be missed.