On 25 June 2010 MacIntyre set his Brooklyn apartment on fire and his body was later found there.
MacIntyre often told people he was orphaned by a Scottish family and raised in an Australian orphanage and a child labor camp. He used the aliases Paul Grant Jeffery, Timothy/Tim C. Allen, Oleg V. Bredikhine, and the nickname Froggy. But a teenage acquaintance alleged that the young MacIntyre spoke then with a plain New York accent from Long Island or Queens, raising questions about his claims of foreign origin. An acquaintance remembers MacIntyre sharing the reason for the "Gwynplaine" in his name; it was, he said, from the film The Man Who Laughs, based on the Victor Hugo novel, in which the title character, Gwynplaine, has had a permanent smile surgically carved on his face. MacIntyre stated that he identified with Gwynplaine and thus chose the name as part of his own.
In 2000, MacIntyre was arrested after a neighbor said he duct-taped her to a chair, shaved her head, and spray-painted her black. He wound up pleading guilty to third-degree misdemeanor assault.
On June 24, 2010, he was removed from his apartment by police and taken to Coney Island Hospital for evaluation after sending a despondent email to friends, one of whom called 911. He was released hours later, and returned home, where he reportedly lit his apartment on fire. The fire "grew quickly into an 'all-hands' blaze that took 12 trucks and 60 firefighters more than an hour to extinguish". A body was removed from the apartment. No other residents of his apartment building were killed. Eventually, the body was positively identified as MacIntyre. After his death, a brother came forward who stated that MacIntyre's life story was fabricated, but did not provide any details about his real-life story or the reasons for his fabrications and affectations.
Although MacIntyre professionally published many works of non-fiction and literature, he is best known as an author of genre fiction: specifically, science fiction, fantasy, horror and mystery stories. His short stories were published in Weird Tales, Analog, Asimov's Science Fiction, Amazing Stories, Absolute Magnitude, Interzone, the Strand Magazine and numerous anthologies, including Terry Carr's Best Science Fiction of the Year #10, Michael Reaves and John Pelan's mystery/horror anthology Shadows Over Baker Street, James Robert Smith and Stephen Mark Rainey's horror anthology Evermore, and Stephen Jones's The Mammoth Book of Best New Horror. For Mike Ashley's The Mammoth Book of Historical Detectives (1995), MacIntyre wrote "Death in the Dawntime," a locked room mystery (or rather, sealed cave mystery) set in Australia around 35,000 BC, which editor Mike Ashley suggests is the furthest in the past a historical whodunnit has been set.
A characteristic of MacIntyre's writing (both fiction and non-fiction) is his penchant for coining new words and resurrecting obscure words. Language authority William Safire acknowledged MacIntyre's neologism of "Clintonym" and quoted his historical etymology research.
In addition to publishing science fiction in Analog, MacIntyre also contributed to that magazine as an artist, illustrating his own stories and one by Ron Goulart.
MacIntyre wrote a considerable number of book reviews for The Magazine of Fantasy & Science Fiction. In the July 2003 issue of that magazine, MacIntyre mentioned that he was related to the wife of Scottish author Eric Linklater. This admission is significant, as MacIntyre had stated (in interviews and at science-fiction conventions) that he was estranged from his abusive family and did not acknowledge them. He had legally changed his name, officially filing a deed poll: "Fergus MacIntyre" was therefore his legal name but not his birth name. He had acknowledged that he took the name "Gwynplaine" from the protagonist of The Man Who Laughs, a novel by Victor Hugo.
MacIntyre claimed to have contributed substantial script material to a 2006 documentary about actress Theda Bara, The Woman with the Hungry Eyes: he claimed his contributions included the film's title and an interview he had conducted with author Fritz Leiber. He is only listed under the "Special Thanks" section of the credits; MacIntyre claimed to be contractually prevented from receiving a screenplay credit.