Cullen grew up in rural North Carolina, but now lives in the St. Louis area with his wife Cindy and his son Jackson. His noir/horror comic (and first collaboration with Brian Hurtt), The Damned, was published in 2007 by Oni Press. The follow-up, The Damned: Prodigal Sons, was released in 2008. In addition to The Sixth Gun, his current projects include Crooked Hills, a middle reader horror prose series from Evileye Books; The Tooth, an original graphic novel from Oni Press; and various work for Marvel and DC.