He created the following series:
- Hawk and Fisher
- Secret Histories
- The Twilight of the Empire (prelude to the Deathstalker series)
- the Forest Kingdom series which is set in the same continuity as Hawk and Fisher.
- Ghost Finders
- The Ishmael Jones Mysteries.
As well as some stand-alone books:
- Drinking Midnight Wine
- Shadows Fall
He also wrote the independent horror film, Judas Ghost, which is in the same setting as most of his books.
As of 2017, after informing readers of his diagnosis with diabetes, Green has announced his intention to wrap up his currently-ongoing novel series, the better to avert Orphaned Series should his health worsen. Short stories featuring the Nightside, the Droods, the Carnacki Institute, and so forth will continue, and characters from past and present series will still make cameos in Green's future stand-alone novels, because "that's what I do".
This writer provides examples of:
- And I Must Scream: Showing people trapped in fates worse than death is one of Green's stock Kick the Dog methods, to make it all the more satisfying when the baddies responsible get their nasty arses kicked.
- Angels, Devils and Squid: Particularly apparent in the Nightside. And never presume that the angels are the friendly option...
- Anthropomorphic Personification: Entities referred to as "Transient Beings" crop up in many of Green's series, and embody concepts or abstractions in the flesh. Not literally Anthropomorphic in this case, as many are non-humanoid.
- Author Appeal: Expect a love of The '60s to crop up in any modern fantasy series he writes.
- Author Catch Phrase: In all three of his urban fantasy series, he uses variations on "Needs must, when the devil drives" and "Suddenly and messily." And when we say variations on the first one, we mean he's gotten pretty damn creative. Also, he loves to have his characters to use their over-the-top reputations to intimidate their foes.
- blank always lies; unless the truth hurts more.
- Why this is hell, nor am I out of it.
- Ah, the old jokes are the best.
- But then, you would say that, wouldn't you.
- There's some shit with which I will not put up.
- ...and it was the easiest thing in the world to [something awesome].
- "[something possibly negative]," they said, not unkindly.
- Green's more recent works tend to feature novel ways of threatening to smack people (e.g. "I've got a slap in my pocket, want to borrow it?").
- References to things other than Nature - entertainment, cuisine, gambling - being "red in tooth and claw" crop up occasionally, as did one instance (in an Alternate Universe) where Nature was "red in thorn and branch" instead.
- Batman Cold Open: All three of the modern fantasy works have employed this, as have the Hawk & Fisher novels.
- Battle Couple: Hawk and Fisher. Eddie and Molly. Taylor and Suzie. Lucy and Betty Coltrane. Pretty much any Drood couple. Jordan and Catriona. Sir Vivian and Cally.
- Body Double: The premise of Blood And Honor.
- Canon Welding: He's managed to tie in almost all of his own personal property without even resorting to parallel dimensions (though those do exist in his works).
- Deus ex Machina: Frequent and unashamed.
- Early Installment Weirdness: Drinking Midnight Wine, the first Greenverse novel to feature characters from our own Earth and era, depicts the real and magical worlds as side-by-side realities (Veritie and Mysterie) that don't interact much, with magic being greatly weakened in the former and science, little-known in the latter. All his later Urban Fantasy works depict the extraordinary as a fully-integrated, albeit secretive component of our world, equally-rich in sorcery, weird science, and the supernatural, that's only kept hidden from the masses by a combination of The Masquerade, Invisible to Normals, and a whole lot of Weirdness Censor.
- Eldritch Abomination: He absolutely loves them. Even more, he loves having his heroes bitch-slap them and laugh in their faces.
- Fantasy Kitchen Sink: Up to Eleven
- Go Mad from the Revelation: Whatever the series, it's always dangerous to use the Sight in the Greenverse, because seeing too much of how the world really is can destroy humans' sanity. Also, it's repeatedly suggested that long ago, humanity was denied our original destiny: one that's completely at odds with anything we've imagined it to be, such that learning what it is either drives us insane or converts us into incomprehensible Starfish Aliens.
- Hand of Glory: A recurring feature of his fantasy works, along with aboriginal pointing bones.
- Names to Run Away from Really Fast: Practically any character, creature, or artifact that rates a title In Capital Letters can be counted on to be powerful, scary, heavy on the magic, or all three. Subverted with John Taylor's deceptively-humdrum name, and the unnamed rainbow sword from the Forest Kingdom novels.
- Our Cryptids Are More Mysterious: If it's a cryptid, it's bound to turn up in one series or another.
- Reality Bleed: Happens in multiple series, usually when entities from "higher" (= more Real) or "lower" (= more Unreal) dimensions trespass where they don't belong.
- Rule of Cool: Done in all of his works
- Shout-Out: Several references to the works of M. R. James turn up in his novels.
- Superman Stays Out of Gotham: While characters from his three Urban Fantasy series (Nightside, Secret Histories, Ghost Finders) have had a few crossover cameos, and numerous back-and-forth references are name-dropped, each series still has to solve its own problems. Thus, we don't see Droods participating in the Lilith War or hunting the Flesh Undying, or John Taylor dropping by to Find ghosts with his Gift. Justified, as the Droods are barred from the Nightside by ancient pact (that doesn't stop a couple from popping in now and then, but they know better than to armor up), the Ghost Finders have a specialized niche of their own, and John Taylor has a lot on his hands especially after ending up with Walker's job.
- Averted, with nigh-cataclysmic consequences all round, in the Nightside/Secret Histories Grand Finale crossover Night Fall.
- The World Is Always Doomed