Clark was born in Canby, Minnesota. He attended South Dakota State University and Moorhead State University in Moorhead, Minnesota, studying history and political science. He became interested in the UFO phenomenon in the 1960s, while he was still in his teen years. He has served as a writer, reporter, and editor for a number of magazines which cover UFOs and other paranormal subjects. Clark is a board member of the Center for UFO Studies, (CUFOS), one of the few civilian UFO research groups with credible scientific support.

After living for many years in the Chicago area, where CUFOS is headquartered, Clark returned to his hometown of Canby, Minnesota, where he currently lives and works. His wife is an editor for Omnigraphics, a publishing company.

Embracing then rejecting paranormal explanations

In the 1970s, Clark embraced some paranormal ideas to explain UFOs and other unusual phenomena. He was influenced by the "ultraterrestrials" theory of John Keel, and the so-called interdimensional hypothesis (which had been championed by Dr. Jacques Vallée). Clark even co-wrote a book on the subject with longtime friend Loren Coleman. Eventually, however, Clark came to reject the paranormal explanations: he thought them unscientific and judged many of their promoters prone to reaching unsupported conclusions and making grand pronouncements without evidence.

Clark wrote his "position statement" for The Encyclopedia of UFOs (Story, 1980, p. 75, emphasis in original):

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 that are purported to have some degree of documentable support...whether physical evidence, or reliably reported events. He has argued very cautiously in favor of the extraterrestrial hypothesis, not as proven fact but as a working hypothesis, choosing to focus on the UFO cases he regards as the most promising: multiple witness and/or UFO cases which are said to leave physical evidence.

In 1983, Clark described himself as a "sceptical Fortean", writing, "Charles Fort was sceptical of establishment humbuggery and so are those of us who follow in his footsteps. That hasn't changed and I hope it never will. But now it's time that we train a sceptical eye on our own humbuggery as well."

From 1976 to 1989 he was the editor of Fate magazine.

Since 1985 Clark has served as the editor of the International UFO Reporter, the official journal of CUFOS. He has also been the editor of the Journal of UFO Studies, the only peer reviewed publication in Ufology.

The UFO Encyclopedia

Perhaps Clark's greatest accomplishment in the field of UFO studies came in the 1990s with the publication of his massive, award-winning UFO Encyclopedia.

The UFO Encyclopedia was first published by respected academic and reference books specialists Omnigraphics as a three-volume hardcover set in the 1990s. In 1997, Visible Ink published an abridged, mass-market trade paperback version under the title The UFO Book, and an updated two-volume hardcover edition of the Encyclopedia was published in 1998. Clark wrote all the hundreds of entries, with a few exceptions, including an essay by biochemist Michael D. Swords about the extraterrestrial hypothesis, one article by folklorist Thomas E. Bullard about the abduction phenomenon, several articles by ufologist Michael Chalker about some Australian UFO incidents, and contributions by UFO researcher Brad Sparks.

Backed by detailed research and extensive bibliographies, Clark's encyclopedia is widely regarded by most UFO researchers, and even many skeptics, as one of the best-researched and most credible publications on the often-controversial subject of UFOs; the Association of College and Research Libraries described the book as "the definitive work on the [UFO] subject for many years to come" while Library Journal notes that one of the judges for Clark's Benjamin Franklin Award declared the UFO Book (a condensed, mass-market version of the UFO Encyclopedia) "an exhaustive, non-judgmental look at the history of unidentified flying objects ... the writing is top notch and clear." Critic Douglas Chapman praises the Encyclopedia as "a treasure for anyone interested in UFOs. The only people unlikely to be pleased by it are dogmatics of any stripe, for multiple points of view are represented." Psychologist Stuart Appelle praises "[Clark's] attempt to maintain objectivity ... in no case is the reader given less than a clear statement of the facts and opinions at hand, and ample opportunity to reach a conclusion on his or her own"; in the Skeptic Files, Chris A. Rutkowski wrote that despite "a definitely 'pro' [UFO] standpoint, [Clark] is wise to include reactions and explanations of major UFO cases by debunkers such as Philip Klass and Donald Menzel. In Clark's telling of the tales, he points out major boners and silly comments by debunkers AS WELL AS overboard proponents, although the former group won't be thrilled by the portrayals ... Otherwise, the UFO ENCYCLOPEDIA is an excellent reference work, and should be added to any library of Fortean material. Readers new to the field should peruse the book to get a 'proper schooling' in the subject" (in a follow-up, Rutkowski stressed "I want to make a special effort to emphasize that my review [of] Clark's UFO Encyclopedia was meant to be very positive, and not negative as some had interpreted)".

Songwriting and music

In addition to his duties as a writer, researcher, and editor, Clark has also written songs which have been recorded or performed by musicians such as Emmylou Harris, Mary Chapin Carpenter, and Tom T. Hall. He has often collaborated with Robin and Linda Williams

Clark has also written numerous reviews of American folk music albums and CDs for Rambles magazine.

An abridged version of the UFO Encyclopedia, entitled The UFO Book, won the 1998 Benjamin Franklin Award in the Science/Environment category from the Independent Book Publishers Association.

Clark is also the 1992 recipient of the Isabel Davis Award (given by the Fund for UFO Research) for promoting rationality in the study of UFOs. He is an active participant in debates and discussions on the "UFO Updates" message boards and website.