Ellison was born to a Jewish family in Cleveland, Ohio, on May 27, 1934, the son of Serita (née Rosenthal) and Louis Laverne Ellison, a dentist and jeweler. He had an older sister, Beverly (Rabnick), who was born in 1926. She died in 2010 without having spoken to him since their mother’s funeral in 1976. His family subsequently moved to Painesville, Ohio, but returned to Cleveland in 1949, following his father’s death. Ellison frequently ran away from home (in an interview with Tom Snyder he would later claim it was due to discrimination by his high school peers), taking an array of odd jobs—including, by age 18, “tuna fisherman off the coast of Galveston, itinerant crop-picker down in New Orleans, hired gun for a wealthy neurotic, nitroglycerine truck driver in North Carolina, short-order cook, cab driver, lithographer, book salesman, floorwalker in a department store, door-to-door brush salesman, and as a youngster, an actor in several productions at the Cleveland Play House”. In 1947, a fan letter he wrote to Real Fact Comics became his first published writing.

Ellison published two serialized stories in the Cleveland News during 1949, and he sold a story to EC Comics early in the 1950s. During this period, Ellison was an active and visible member of science fiction fandom, and published his own science fiction fanzines, such as Dimensions (which had previously been the Bulletin of the Cleveland Science Fantasy Society for the Cleveland Science Fantasy Society, and later Science Fantasy Bulletin.) Ellison moved to New York City in 1955 to pursue a writing career, primarily in science fiction. Over the next two years, he published more than 100 short stories and articles. The short stories collected as Sex Gang — which Ellison described in a 2012 interview as “mainstream erotica” — date from this period.

Ellison married five times; each relationship ended within a few years, except the last. His first wife was Charlotte Stein, whom he married in 1956. They divorced in 1960, and he later described the marriage as “four years of hell as sustained as the whine of a generator.” Later that year he married Billie Joyce Sanders; they divorced in 1963. His 1966 marriage to Loretta Patrick lasted only seven weeks. In 1976, he married Lori Horowitz. He was 41 and she was 19, and he later said of the marriage, “I was desperately in love with her, but it was a stupid marriage on my part.” They were divorced after eight months. He and Susan Toth married in 1986, and they remained together, living in Los Angeles, until his death 32 years later. Susan died in August 2020.

He served in the U.S. Army from 1957 to 1959. His first novel, Web of the City, was published during his military service in 1958, and he said that he had written the bulk of it while undergoing basic training at Fort Benning, Georgia. After leaving the army, he relocated to Chicago, where he edited Rogue magazine.

Ellison moved to California in 1962, and subsequently began to sell his writing to Hollywood. He co-wrote the screenplay for The Oscar (1966), starring Stephen Boyd and Elke Sommer. Ellison also sold scripts to many television shows: The Loretta Young Show (using the name Harlan Ellis),The Flying Nun, Burke’s Law, Route 66, The Outer Limits, Star Trek, The Man from U.N.C.L.E., Cimarron Strip, and The Alfred Hitchcock Hour. Ellison’s screenplay for the Star Trek episode “The City on the Edge of Forever” has been considered the best of the 79 episodes in the series.

In 1965, he participated in the Selma to Montgomery marches led by Martin Luther King, Jr.

In 1966, in an article that Esquire magazine would later name as the best magazine piece ever written, the journalist Gay Talese wrote about the goings-on around Frank Sinatra. The article, entitled “Frank Sinatra Has a Cold”, briefly describes a clash between the young Harlan Ellison and Frank Sinatra, when the crooner took exception to Ellison’s boots during a billiards game.

Ellison was among those who in 1968 signed an anti-Vietnam War advertisement in Galaxy Science Fiction. In 1969, Ellison was Guest of Honor at Texas A&M University’s first science fiction convention, Aggiecon, where he reportedly referred to the university’s Corps of Cadets as “America’s next generation of Nazis”, inspired in part by the continuing Vietnam War. Although the university was no longer solely a military school (from 1965), the student body was predominantly made up of cadet members. Between Ellison’s anti-military remarks and a food fight that broke out in the ballroom of the hotel where the gathering was held (although, according to Ellison in 2000, the food fight actually started in a Denny’s because the staff disappeared and they could not get their check), the school’s administration almost refused to approve the science fiction convention the next year and no guest of honor was invited for the next two Aggiecons. However, Ellison was subsequently invited back as Guest of Honor for Aggiecon V (1974).

The Last Dangerous Visions (TLDV), the third volume of Ellison’s anthology series, was originally announced for publication in 1973, but was never published. Nearly 150 writers (many now dead) submitted works for the volume. In 1993, Ellison threatened to sue New England Science Fiction Association (NESFA) for publishing “Himself in Anachron”, a short story written by Cordwainer Smith and sold to Ellison for the book by his widow, but later reached an amicable settlement.

Ellison’s original script was first published in the 1976 anthology Six Science Fiction Plays, edited by Roger Elwood. The aired version was adapted for the Star Trek Fotonovel series in 1977. In 1995, Borderlands Press published The City on the Edge of Forever, with nearly 300 pages, comprising an essay by Ellison, four versions of the teleplay, and eight “Afterwords” contributed by other parties. He greatly expanded the introduction for the paperback edition, in which he explained what he called a “fatally inept” treatment.

In a 1980 lawsuit against ABC and Paramount Pictures, Ellison and Ben Bova claimed that the TV series Future Cop was based on their short story “Brillo”, winning a $337,000 judgement.

In his 1981 book about the horror genre, Danse Macabre, Stephen King reviewed Ellison’s collection Strange Wine and considered it one of the best horror books published between 1950 and 1980.

He was awarded the Silver Pen for Journalism by International PEN, the international writers’ union, in 1982. In 1990, Ellison was honored by International PEN for continuing commitment to artistic freedom and the battle against censorship. In 1998, he was awarded the “Defender of Liberty” award by the Comic Book Legal Defense Fund.

In 1983, Marvel Comics released The Incredible Hulk #286, entitled “Hero”, written by Bill Mantlo. Three issues later, Marvel put up a letter claiming that Mantlo adapted “Soldier” for use as a Hulk story, but they forgot to credit Ellison and had it pointed out by readers. In actuality, then-Editor-in-Chief Jim Shooter signed off on the story, not having seen the Outer Limits episode it was based on and not realizing Mantlo copied it wholesale. The day the issue went to stands, he was contacted by an angry Ellison, who calmed down after Shooter admitted the error. Although he could have claimed hundreds of thousands of dollars in damages, Ellison only requested the same payment Mantlo got for the story, writer’s credit and a lifetime subscription to everything Marvel published.

In 1985 Ellison allegedly publicly assaulted author and critic Charles Platt at the Nebula Awards banquet. Platt did not pursue legal action against Ellison, and the two men later signed a “non-aggression pact”, promising never to discuss the incident again nor to have any contact with one another. Platt claims that Ellison often publicly boasted about the incident.

Ellison won eight Hugo Awards, a shared award for the screenplay of A Boy and his Dog that he counted as “half a Hugo”, and two special awards from annual World SF Conventions; four Nebula Awards of the Science Fiction and Fantasy Writers of America (SFWA); five Bram Stoker Awards of the Horror Writers Association (HWA); two Edgar Awards of the Mystery Writers of America; two World Fantasy Awards from annual conventions; and two Georges Méliès fantasy film awards. In 1987, Ellison was awarded the Inkpot Award.

Ellison’s short story “The Man Who Rowed Christopher Columbus Ashore” (1992) was selected for inclusion in the 1993 edition of The Best American Short Stories.

Ellison won the World Fantasy Award for Life Achievement in 1993. HWA gave him its Lifetime Achievement Award in 1996 and the World Horror Convention named him Grand Master in 2000. He was awarded the Gallun Award for Lifetime Achievement in Science Fiction from I-CON in 1997.

In 1994, he suffered a heart attack and was hospitalized for quadruple coronary artery bypass surgery. From 2010, he received treatment for clinical depression.

Ellison’s official website, harlanellison.com, was launched in 1995 as a fan page; for several years, Ellison was a regular poster in its discussion forum.

In March 1998, the National Women’s Committee of Brandeis University honored him with their 1998 Words, Wit, Wisdom award.

On April 24, 2000, Ellison sued Stephen Robertson for posting four stories to the newsgroup “alt.binaries.e-book” without authorization. The other defendants were AOL and RemarQ, an internet service provider who owned servers hosting the newsgroup. Ellison alleged they had failed to halt copyright infringement in accordance with the “Notice and Takedown Procedure” outlined in the 1998 Digital Millennium Copyright Act. Robertson and RemarQ first settled with Ellison, and then AOL likewise settled with Ellison in June 2004, under conditions that were not made public. Since those settlements Ellison initiated legal action or takedown notices against more than 240 people who have allegedly distributed his writings on the Internet, saying, “If you put your hand in my pocket, you’ll drag back six inches of bloody stump”.

Ellison was named 2002’s winner of the Committee for the Scientific Investigation of Claims of the Paranormal’s “Distinguished Skeptic Award”, in recognition of his contributions to science and critical thinking. Ellison was presented with the award at the Skeptics Convention in Burbank, California, on June 22, 2002.

On September 20, 2006, Ellison sued comic book and magazine publisher Fantagraphics, stating they had defamed him in their book Comics As Art (We Told You So). The book recounts the history of Fantagraphics and discussed a lawsuit that resulted from a 1980 Ellison interview with Fantagraphics’ industry news magazine, The Comics Journal. In this interview Ellison referred to comic book writer Michael Fleisher, calling him “bugfuck” and “derange-o”. Fleisher lost his libel suit against Ellison and Fantagraphics on December 9, 1986.

SFWA named him its 23rd Grand Master of fantasy and science fiction in 2006 and the Science Fiction Hall of Fame inducted him in 2011. That year he also received the fourth J. Lloyd Eaton Lifetime Achievement Award in Science Fiction, presented by the UCR Libraries at the 2011 Eaton SF Conference, “Global Science Fiction”.

In September 2007, Ellison made his last public appearance in his hometown of Cleveland, Ohio, for the Midwestern debut of the documentary Dreams with Sharp Teeth at the Cleveland Public Library.

Ellison, after reading unpublished drafts of the book on Fantagraphics’s website, believed that he had been defamed by several anecdotes related to this incident. He sued in the Superior Court for the State of California, in Santa Monica. Fantagraphics attempted to have the lawsuit dismissed. In their motion to dismiss, Fantagraphics argued that the statements were both their personal opinions and generally believed to be true anecdotes. On February 12, 2007, the presiding judge ruled against Fantagraphics’ anti-SLAPP motion for dismissal. On June 29, 2007, Ellison claimed that the litigation had been resolved pending Fantagraphics’ removal of all references to the case from their website. No money or apologies changed hands in the settlement as posted on August 17, 2007.

On March 13, 2009, Ellison sued CBS Paramount Television, seeking payment of 25% of net receipts from merchandising, publishing, and other income from the episode since 1967; the suit also names the Writers Guild of America for allegedly failing to act on Ellison’s behalf. On October 23, 2009, Variety magazine reported that a settlement had been reached.

In December 2009, Ellison was nominated for a Grammy award in the category Best Spoken Word Album For Children for his reading of Through the Looking-Glass And What Alice Found There for Blackstone Audio, Inc.

As of 2013, Ellison is the only three-time winner of the Nebula Award for Best Short Story. He won his other Nebula in the novella category.

In 2014 Ellison made a guest appearance on the album Finding Love in Hell by the stoner metal band Leaving Babylon, reading his piece “The Silence” (originally published in Mind Fields) as an introduction to the song “Dead to Me.”