He was born in Manhattan, New York City on April 27, 1901. He grew up in the Harlem area of Manhattan. His father was a prosperous dentist and his mother was May Doty. The family resided at 823 West End Avenue in Manhattan. Long's father was a keen fisher and hunter, and Long accompanied the family on annual summer vacations from the age of six months to 17, usually in the Thousand Islands region on the Canadian shore, about seven miles from the village of Gananoque. When he was three years old, on one of these vacations, Long fell into the river at the end of a long pier and contracted pneumonia 
A lifelong resident of New York City, Long was educated in the New York City public school system. As a boy he was fascinated by natural history, and wrote that he dreamed of running "away from home and explore the great rain forests of the Amazon." He developed his interest in the weird by reading the Oz books, Jules Verne, and H.G. Wells as well as Ambrose Bierce and Edgar Allan Poe. Though writing was to be his life's work, he once commented that as "important as writing is, I could have been completely happy if I had a secure position in a field that has always had a tremendous emotion and an imaginative appeal for me—that of natural history."
In his late teens, he was active in the United Amateur Press Association (UAPA) in which he won a prize from The Boy's World (around 1919) and thus discovered amateur journalism. His first published tale was "Dr Whitlock's Price (United Amateur, March 1920). Long's story "The Eye Above the Mantel" (1921), a pastiche of Edgar Allan Poe, in UAPA, caught the eye of H. P. Lovecraft, sparking a friendship and correspondence that would endure until Lovecraft's death in 1937.
Long attended New York University from 1920 to 1921, studying journalism but later transferred to Columbia, leaving without a degree. In 1921, he suffered a severe attack of appendicitis, leading to a ruptured appendix and peritonitis. He spent a month in New York's Roosevelt Hospital, where he came close to dying. Long's brush with death propelled him into a decision that he would leave college to pursue a freelance writing career.
Early career: the 1920s
In 1924, at the age of 22, he sold his first short story, "The Desert Lich", to Weird Tales magazine. Throughout the next four decades, Long was to be a frequent contributor to pulp magazines, including two of the most famous: Weird Tales (under editor Farnsworth Wright) and Astounding Science Fiction (under editor John W. Campbell). Long was an active freelance writer, also publishing many non-fiction articles.
His first book, the scarce volume A Man from Genoa and Other Poems, was published in 1926 by W. Paul Cook. Two copies are held in the collections of  The poems in this collection won praise from a great variety of writers, among them Arthur Machen, Robinson Jeffers, William Ellery Leonard, John Drinkwater, John Masefield and George Sterling. Samuel Loveman declared that Long's poem "The Marriage of Sir John de Mandeville" was worthy of Christopher Marlowe.
Long's closest friends (apart from H. P. Lovecraft) in this period included Samuel Loveman, H. Warner Munn, and James F. Morton. He had several encounters with Hart Crane, who lived one flight above Loveman in Brooklyn Heights.
"The Horror from the Hills", a story serialised in 1931 in Weird Tales, incorporated almost verbatim a dream H. P. Lovecraft related to him (among other correspondents) in a letter. The short novel was published many years later in separate book form by Arkham House in 1963, as The Horror from the Hills.
In the late 1930s, Long turned his hand to science fiction, writing for Astounding Science Fiction. He also contributed horror stories to Unknown, later called Unknown Worlds. Long contributed an episode (along with C.L. Moore, Robert E. Howard and H. P. Lovecraft) to the round-robin story "The Challenge from Beyond" (1935).
Like The Man from Genoa and Other Poems, his second book is a volume of fantastic verse: The Goblin Tower (1935), published jointly by H. P. Lovecraft and Robert H. Barlow under Barlow's The Dragonfly Press imprint. (A variant edition of this volume was published in 1945 by New Collectors Group - see Bibliography). Published in an edition of only 100 copies, this volume is exceedingly scarce; two copies are held at the collections of John Hay Library.
In pulps such as Thrilling Wonder Stories and Startling Stories during the 1940s, Long sometimes wrote using the pseudonym 'Leslie Northern.' What Long characterized as a "minor disability" kept him out of World War II and writing full-time during the early 1940s.
Long reputedly ghost-wrote two, possibly three, of the Ellery Queen Junior novels (see Ellery Queen (house name) (mentioned in correspondence with August Derleth) but unfortunately did not identify the three titles. It has been speculated by researchers that the two are: The Golden Eagle Mystery (1942) and The Green Turtle Mystery (1944). The third one may have been the fugitive The Mystery of the Golden Butterfly, which was apparently never published. (This volume is mentioned as Long's on the rear panel of The Horror from the Hills and on the rear flap of The Rim of the Unknown).
He wrote comic books in the 1940s, including horror stories for Adventures Into the Unknown (ACG). Long contributed several original scripts to this comic's early issues, as well as an adaptation of Walpole's The Castle of Otranto. He authored scripts for Planet Comics, Superman, Congo Bill, DC's Golden Age Green Lantern, and the Fawcett Comics Captain Marvel. He worked in the 1940s as a script-reader for Twentieth Century Fox  Long wrote crime and weird menace stories for Ten Gang Mystery and other magazines.
During the 1940s, Long lived for a period in California.
Long credited Theodore Sturgeon, whom he met several times in the mid-1940s, as being instrumental in getting one of his middle-period stories, "A Guest in the House", produced on CBS-TV in 1954.
In 1946, Arkham House published Long's first collection of supernatural fiction, The Hounds of Tindalos, which collected 21 of his best tales from the previous twenty years of magazine publication. It featured works which had appeared in such pulps as Weird Tales, Astounding Stories, Super Science Stories, Unknown, Thrilling Wonder Stories, Dynamic Science Fiction, Startling Stories, and others. In "The Man from Time", a time-traveller from the future has an encounter with writer F. Scott Fitzgerald.
His later science fiction works include the story collection John Carstairs, Space Detective (1949) about a 'botanical detective', and the novels Space Station 1 (1957), Mars is My Destination (1962) and It Was the Day of the Robot (1963).
Long's 1935 Weird Tales
story "The Body-Masters" was reprinted in 1950 as "The Love-Slave and the Scientists".
In the 1950s he was involved with editing five different magazines. He was uncredited associate editor on The Saint Mystery Magazine and Fantastic Universe. He was associate editor on Satellite Science Fiction, 1959; on Short Stories, 1959–60; and on Mike Shayne Mystery Magazine until 1966.
Long several times met fellow Weird Tales writer and poet Joseph Payne Brennan, and later provided the foreword for Brennan's The Chronicles of Lucius Leffing (1977).
After the decline of the pulps, Long moved into the prolific production of science fiction and gothic romance novels during the 1960s and 1970s. He even wrote a Man from UNCLE story, "The Electronic Frankenstein Affair", which appeared under the pen name Robert Hart Davis in the Man from UNCLE Magazine.
In 1960, he married Lyda Arco, an artists' representative and aficionado of drama. She was a Russian descended from a line of actors in the Yiddish theatre who ran a salon in Chelsea, NY. They stayed together till Long's death in 1994, but had no children. Long described himself as an "agnostic." Referring to Lovecraft, Long wrote that he "always shared HPL's skepticism . . . concerning the entire range of alleged supernatural occurrences and what is commonly defined as 'the occult.'"
In 1963 Arkham House published Long's novel The Horror from the Hills, a work partly incorporating Lovecraft's account of a dream Lovecraft had experienced. This work introduced Long's alien entity Chaugnar Faugn into the Cthulhu Mythos cycle.
In 1972 Arkham House published The Rim of the Unknown, their second hardcover collection of Long's work - a volume focussing primarily on his science fiction short stories.
Long wrote nine modern Gothic novels, starting with So Dark a Heritage in 1966 (published under his own name), eight of which were published as by "Lyda Belknap Long", a combination of his wife (Lyda Arco Long)'s first name and his middle name and surname. Seven of these appeared during the 1970s; all were entirely his own work and were workmanlike products intended to support him and his wife rather than to be of high literary quality.
Illumination on Long's own life and work is provided by his extensive introduction to The Early Long (1975), a collection of his best early stories which essentially duplicates the contents of The Hounds of Tindalos but to which Long adds detailed headnotes to each story. Further writing on his own life is found in his Autobiographical Memoir (Necronomicon Press, 1986).
Long's book-length memoir of H. P. Lovecraft, Howard Phillips Lovecraft: Dreamer on the Nightside, was issued by Arkham House in 1975. It was written in haste as a result of Long's reading of L. Sprague de Camp's Lovecraft: A Biography (1975), which Long felt to be biased against Lovecraft.
In 1977, Arkham House issued Long's hardcover poetry collection In Mayan Splendor, containing all the poems from A Man from Genoa and Other Poems (1924) and The Goblin Tower (1926). The same year he won the First Fandom Hall of Fame award (1977). In 1978 he won the World Fantasy Award for Life Achievement (at the 1978 4th World Fantasy Convention).
Later career: 1980s–1990s
Long's literary output slowed down after 1977, with his gothic The Lemoyne Heritage. He published several scattered stories in the 1980s including the story chapbook "Rehearsal Night" (Pub: Thomas L. Owen,1981) and one episode in the round-robin sequence Ghor Kin-Slayer (Necronomicon Press, 1997). He and his wife lived in extreme poverty during the 1980s and 1990s in an apartment in Chelsea, Manhattan - a period documented in Peter Cannon's memoir Long Memories (1997).
In 1987, Long was awarded the Bram Stoker Award for Lifetime Achievement (from the Horror Writers Association).
Long, though confined to a wheelchair, was a Guest of Honour at the H.P. Lovecraft Centennial Conference in Providence, Rhode Island, in 1990, where he spoke on panels regarding his memories of his great friend and literary mentor.
Long died of pneumonia on January 3, 1994 at the age of 92 at Saint Vincent's Catholic Medical Center in Manhattan, after a seven-decade career as a writer and editor. He was briefly survived by his wife, Lyda.
Due to his poverty, he was interred in a potter's field for indigents. Friends and colleagues, on learning of this indignity, had his remains moved and reinterred at New York City's Woodlawn Cemetery, in a family plot near that of Lovecraft's grandparents. A graveside ceremony on Nov 3 1995 was attended by such figures as Scott D. Briggs, Peter Cannon, Stefan Dziemianowicz, Ben P. Indick, S. T. Joshi, T.E.D. Klein and others and with a poignant homily delivered by the Rev. Robert M. Price. On Nov 17, 1995 the actual interment of Long's body took place, an event witnessed by Peter Cannon, Ben P. Indick and S. T. Joshi. Long's fans contributed over $3,000 to have his name engraved upon the central shaft of his burial plot. Lyda died shortly after Frank; her ashes were scattered on his grave.