Brennan's first professional sale came in December 1940 with the publication of the poem "When Snow Is Hung", which appeared in the Christian Science Monitor Home Forum, and he continued writing poetry up until the time of his death.
Several of his short story collections concern an occult detective named Lucius Leffing in the vein of Carnacki and Algernon Blackwood's John Silence.
"[6] Brennan has stated in numerous autobiographical snippets that a chance encounter with the collected works of Edgar Allan Poe is what sparked his interest and ambition to engage in writing himself.
[6] Shortly after he was born in Bridgeport, CT, in 1917 (the same year of birth as his fellow Weird Tales writer Robert Bloch), Brennan's family moved back to New Haven, where he lived thereafter.
Later he used, with varying degrees of rapport and success, Laurence R. D'Orsay, Jack Schaffner, the Scott Meredith Literary Agency, Kirby McCauley, Kenneth S. White and R. Dixon Smith.
Recognized editors of his work include Dorothy McIlwraith, Frank Belknap Long, Charles L. Grant, Peter Haining, Helen Hoke, Robert Arthur, Les Daniels, August Derleth, Ruth Iodice, Lilith Lorraine, Gustav Davidson, F. E. S. Finn, Stuart David Schiff], Gerald W. Page, George Abbe, and Loring Williams.
Among the many poets who contributed over the years were George Abbe, Duane Ackerson, Doris Philbrick Brennan, Judson Crews, August Derleth, Alan Donovan, Alfred Dorn, Janet Fox (Scavenger's Newsletter), Skip Galloway, Joseph Joel Keith, Lilith Lorraine, Joseph Francis Murphy, Rebecca Newth, William J. Noble, Lori Petri, Dorothy Quick, Sydney King Russell, Wade Wellman (son of Manly Wade Wellman), Mary Winter, and Celeste Turner Wright.
The environs of Yale University were closely bound up with Long's ancestral heritage - his maternal forebears the Manfields were among the city's earliest settlers, and this gave him much in common with Brennan.
Much later Brennan was also collected in the Zebra Books revival of the magazine, Weird Tales #2, with the Leffing case "The Nursing Home Horror", retitled "Fear".
According to Donald M. Grant, "together with its companion magazine, Essence, it (Macabre) provided the opportunity and the encouragement for publication of poems and stories by writers seeking recognition in a period that lacked a vehicle for development.
Other contributions included "Wei-Thogga" by Mike Ambrose, "Ice People" by George Dendrinos, "Balthor the Dreamer" by W. Paul Ganley, "The Floating Coffin" by John Perry, and "Day of Departure" by Frank Sherry.
[12] The Macabre House imprint also published most of Brennan's horror fiction in the 1950s to early 1970s, such as the volumes The Dark Returners (1959), Scream at Midnight (1963) and The Casebook of Lucius Leffing (1973).
But in his scholarly reserve and quietness there is a sagacity of a high order, a brilliance that blazes and sears and shatters the horrific as if it were a vessel of glass with the deadly precision of a rapier thrust.
He also became a member of the Praed Street Irregulars, a society founded by fellow author Luther Norris in honor of August Derleth's sleuth Solar Pons.
Long says of Brennan that he was a "storyteller of exceptional gifts who trusts his own creation right up to the hilt" and that "no present-day writer of fantasy conforms, in quite so miraculous a way, to the most fascinating of literary legends - that of the cultivated poet-philosopher-scholar who explores literature's most adventurous byways, in thrall to the darkly mysterious and the subtly terrifying".
The 1981 poetry collection Creep to Death assembled an even-handed blend of 84 poems by Brennan, many culled from the pages of Essence and Macabre, and various contemporary semi-pro genre magazines of the day, including Bleak December, Cross Plains, Myrddin, Nyctalops, Weirdbook, Whispers, and the one-off collaboration Toadstool Wine (1975).
In 1982, the short hardboiled detective novel Evil Always Ends made its hardcover debut at the 1982 World Fantasy Convention, at which tribute was paid to Brennan as Guest of Honor.
[1] In 1984, Twilight Zone magazine featured a seven-page spread of Brennan's poems with illustrations—probably the largest such periodical coverage in the history of the fantastic poetry genre.
Many of Brennan's best tales are set within the environs of New Haven and East Hartland, some within the mythical New England town of Juniper Hill, and feature seemingly semi-autobiographical elements throughout.
As Alan Warren points out, many of Brennan's tales involve ghosts or apparitions that make frightening, unexpected appearances in old houses, hospital rooms, or even, as in 'The Man in Grey tweeds", on the highway.
Alan Warren considers that almost singlehandedly, he continued the Weird Tales tradition of well-wrought and atmospheric Gothic horror that seemed moribund for many years until Stephen King, and others employing many of the same methods, arrived on the scene.
"[17] By contrast, S. T. Joshi writes that Brennan has "the ability to devise a clever supernatural idea but an utter deficiency of literary talent to execute it competently.
The hallmark of Brennan's work is an almost childishly simple, unadorned prose that might be thought to facilitate the subtle incursion of the weird; but in reality this flatness of style renders his conceptions preposterous and absurd because of an insufficiency of atmospheric preparation."
Joshi does consider that "Brennan is probably a better poet than a fiction writer, and his simplicity of utterance can be highly effective in short, pungent poems of fantasy and terror.
"[18] Stefan Dziemanowicz comments of the collection Nine Horrors and a Dream that "All the stories in the book are notable for their simple, unaffected style, and their depiction of ordinary suburban and rural people contending with eruptions of the supernatural in their everyday lives."