In 2002, Nickell was one of a number of experts asked by scholar Henry Louis Gates Jr. to evaluate the authenticity of the manuscript of Hannah Crafts' The Bondwoman's Narrative (1853–1860), possibly the first novel by an African-American woman.

"[9] Nickell has worked professionally as a stage magician, carnival pitchman, private detective, blackjack dealer, riverboat manager, university instructor, author, and paranormal investigator, and he lists more than 1,000 personae on his website.

[citation needed] He was profiled by The New Yorker writer Burkhard Bilger, who met Nickell during the summer of 2002 at Lily Dale, New York.

[15]He served as a character consultant to Hilary Swank in her starring role in the horror film The Reaping (2007), in which she plays a paranormal investigator.

He updated the book in 1998 with more recent historical, iconographic, forensic, physical and chemical evidence, with special explanations of the radiocarbon dating process.

For each incident, Nickell reviews the contemporaneous written accounts, explores various natural explanations, explains the cultural environment surrounding the events, and speculates on the motivations of the affected religious community.

He said that although no icon in history has ever been proven authentic in the sense of displaying such attributes, he approaches each case with a suspension of disbelief: "I'm interested in the evidence because I want us to know what the truth is...

From the Virgin Mary's face appearing on a grilled cheese sandwich, to the Cross's regeneration after pieces were removed, to the structural deficiencies of the Loretto Chapel staircase, Nickell's described fact and myth are presented with clarity and respect.

Equally dismissive is his approach to glossolalia (speaking in tongues)... he does not bring to bear the available and abundant data that support the reality of this spiritual gift.

Nickell explains forgeries of Daniel Boone's musket, Mark Hofmann's Mormon papers, and the Vinland Map.

[23] According to Publishers Weekly, Crime Science: Methods of Forensic Detection (1998) provided extensive basic information, with brief case studies.

He and his collaborator, John F. Fisher, look for the answers to the Crystal Skulls, spontaneous human combustion, the Mackenzie House, and lesser known mysteries.

[28] In Entities: Angels, Spirits, Demons, and Other Alien Beings, Nickell shows the development of ghost stories since the 17th century, and how they have been influenced by changing technology and communication methods.

The Outer Edge: Classic Investigations of the Paranormal is a collection of articles edited by Nickell, Barry Karr and Tom Genoni.

It features Nickell and John F. Fischer's 1987 article, "Incredible Cremations: Investigating Spontaneous Combustion Deaths," along with essays by Martin Gardner, Ray Hyman, Susan Blackmore, and James Randi.

He includes an analysis of 21st-century paranormal investigators, particularly Jason Hawes and Grant Wilson of the Syfy Channel's Ghost Hunters.

Nickell's 2005 update of Ambrose Bierce, Unsolved History: Investigating Mysteries of the Past, is the same text with the addition of two books to its "Recommended Works".

"[30] In the case of West Virginia's Mothman, Nickell interviewed witnesses and conducted on-site experiments to find the most likely explanation for the original sightings.

[31] Harry Eager of the Maui News calls Secrets of Sideshows "... virtually an encyclopedia of that nearly extinct form of entertainment.

Author Ed Grabianowski summarizes one of the many possible explanations for lake monster sightings, ... a convincing argument based, again, on data mapping.

Nickell traces the monsters' iconography from first reports to latest sightings, concluding that the tales reflect the evolution of their cultural environment, not any basis in fact.

Rooks as, "... a 200-year, biographical tour of some of the more famous shenanigans and side show splendors of both sincere and charlatan magicians ... {that} guides readers on a fascinating exposé of magical history that leaves us, at the end of every page, thinking, "A-ha!

"[36] The Shroud of Turin, claimed to be the burial cloth of Jesus miraculously imprinted with the image of his crucified body, is one of Christianity's most famous icons.

The Roman Catholic Church, in possession of the Shroud since 1983, has allowed several public viewings and encourages devotions to the image, but takes no official position on the icon's authenticity.

[26]Although Nickell rejects the term "debunker" to describe his work, his evidenced-based investigations of paranormal events have not yet uncovered any miracles, ghosts or monsters.

After an on-air threat of violence from Ed Warren, Nickell stated: I've investigated haunted houses for some twenty years.

[40]Nickell proposes that alien encounters are the result of misinterpreted natural phenomena, hoaxes, or a fantasy prone personality.

[41]Nickel has written the "Investigative Files" column for the Skeptical Inquirer (SI) magazine since 1995 and contributes frequently to the Center for Inquiry website.

The articles reflect the range of Nickell's interests and investigative skills, including spontaneous human combustion, ghost photographs, reincarnation, voodoo, Bigfoot, quack medicine, Elvis, psychic frauds, and phrenology.

[42] Nickell's writing for the Center for Inquiry (CfI) includes "Nickell-odeon Reviews", written with an emphasis on the facts behind the scripts.