Since the late 1980s, Constantine has written more than 20 novels, plus several non-fiction books. She 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 her own works, those of new writers, and those of well-known genre writers, mainly from the UK.