Cullum attended David Lipscomb University in Nashville from 1981 to 1983, and graduated from Abeliene Christian University (Abeliene, Texas) in 1985. He then began his tenure with the News. During that time, he began a daily sydicated comic strip for King Features called Walnut Grove, which appeared in 140 daily newspapers from 1991 until Cullum retired the strip in 2000. Cullum resigned from the News in 1996, and was succeeded by Scott Stantis as editorial cartoonist. Cullum left to pursue other interests; namely, studying in England at Bristol University, and in Scotland at University of St Andrews. He was then able to go to Christ Church College at the University of Oxford in England.
In 2001, Cullum returned to Abelene Christian University where he is a history professor.
- Knox, Melanie J. (October 8, 2003) "Professor Mark Cullum integrates a variety of interests into his career to continually seek challenges." ACU Optimist.