Born in London, Dash attended Peterhouse, a college at the University of Cambridge particularly noted for teaching history. He did postgraduate work at King's College London, and holds a PhD in naval history for the thesis British Submarine Policy 1853—1918.

Dash worked for 20 years as contributing editor and publisher of the journal Fortean Times, which specialises in the study and research of strange and anomalous phenomena. He is regarded as one of the most serious modern investigators into such phenomena, preferring the use of first hand sources and other historically accurate information. His works investigate misconceptions and errors in the field of strange phenomena, typically finding a middle path between "believers" (who, in Dash's view, often accept extraordinary or unusual claims without investigation or on scant evidence) and skeptics and debunkers (who, in Dash's view, often overlook interesting or valuable evidence in their seeming desire to disprove alleged paranormal or anomalous phenomena). In Borderlands (1997), Dash repeatedly notes that researchers of unusual phenomena must always be wary of potential hoaxes; time and again people have perpetrated elaborate pranks or hoaxes with little motive other than to fool others. Overall, he adheres to the psychosocial school of thought, arguing that the vast majority of strange experiences have their origins in the human brain, mediated by the culture in which the percipients were raised.

As a historian, Dash is noted for the high quality of his research, which the academic journal The Age of Sail described as "to a level rarely seen in books intended for a general audience." The Toronto Globe and Mail applauded his work as “up-close, personal, and full of you-are-there detail," adding, "Dash is that rarity: a perfectionist in his research and a writer who perfectly carves out his story with a pen as sharp as a stiletto," and the New York Times praised his "unabashedly cinematic flair, backed by meticulous research." On the other hand, Clive Ponting, writing in the local London paper the Hampstead & Highgate Express, described Borderlands as the work of "a writer on that journal for the gullible, Fortean Times," concluding that the book was "bad, in an unexpected way."

Dash has published seven books and a number of articles and papers in Fortean Times and later spin-off titles such as the academic annual Fortean Studies, published from 1994 through 2000. His most recent title, The First Family, is a new history of Giuseppe Morello and the establishment of the Mafia in the United States.