Clark attended South Dakota State University and Moorhead State University, studying history and political science. He has been an editor of FATE magazine, International UFO Reporter, and board member of the Center for UFO Studies (CUFOS). He has also been the editor of the Journal of UFO Studies, the only peer reviewed publication in ufology.
Clark is widely regarded as one of the most prominent and reputable writers in ufology today. In his Saucer Smear, longtime ufologist James W. Moseley wrote Clark “is acknowledged … as the UFO Field’s leading historian.”
Clark’s greatest accomplishment in the field has been the publication of his comprehensive The UFO Encyclopedia: The Phenomenon from the Beginning (1991). Backed by detailed research and extensive bibliographies, Clark’s encyclopedia is largely considered one of the best-researched and most credible publications on the subject. In 1997 Clark authored The UFO Book, an abridged version of The UFO Encyclopedia, which won the 1998 Benjamin Franklin Award in the Science/Environment category.
Clark wrote his “position statement” in The UFO Encyclopedia: “In the past two or three years, I have become an agnostic about all UFO theories. I have discovered, as one who is no less guilty of it than anyone else, that one can “prove” just about anything by focusing on certain data and ignoring others. I happen to sympathize with the impulse to theorize about UFOs; after all, theories are how we make sense of things. But we ought not under any circumstances to take our theories too seriously, and we must never give them greater primacy than we give the observed facts … In my darker moments, I have come to suspect that UFOs may represent something so far beyond us that our attempts to understand them may be comparable to an ant’s efforts to comprehend the principles of nuclear physics.”
In the years since, Clark has championed a sort of open-ended agnosticism, choosing to focus on phenomena which have some degree of documentable support—whether physical evidence, or reliably reported events and has argued cautiously in favor of the Extraterrestrial Hypothesis (ETH).