Since the late 1980s Constantine has written more than 20 novels, plus several non-fiction books. She features in the Goth Bible and is often included in discussions of alternative sexuality and gender in science fiction and fantasy; many of her novels include same-sex relationships or hermaphrodites or other twists of gender. Magic, mysticism and ancient legends (like the Grigori) also figure strongly in her works.
In 2003 she launched Immanion Press, based out of Stafford, England. The publishing company publishes not only her own works but those of new writers, as well as well-known genre writers, mainly from the UK.