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. His family subsequently moved to Painesville, Ohio, but returned to Cleveland in 1949, following his father's death. Ellison frequently ran away from home, 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".

Robert Silverberg named a character in his first novel, Revolt on Alpha C, for Ellison, who was a neighbor of Silverberg in New York at the time he was writing the book. This was confirmed in a special edition on the occasion of Silverberg's 35th year in the Business.

I Have No Mouth & I Must Scream was included in American Fantastic Tales, volume II (from the 1940s to now), edited by Peter Straub and published by the Library of America in 2009. The Best American Mystery Stories of the Century edited by Tony Hillerman and Otto Penzler (Houghton Mifflin, 2000) included Ellison's "The Whimper of Whipped Dogs."

Ellison published two stories in the Cleveland News during 1949, and he sold a story to EC Comics early in the 1950s. 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. He married Charlotte Stein in 1956, but they divorced four years later. He said of the marriage, "four years of hell as sustained as the whine of a generator."

Ellison attended The Ohio State University for 18 months (1951–53) before being expelled. He has said the expulsion was for hitting a professor who had denigrated his writing ability, and over the next twenty or so years he sent that professor a copy of every story he published.

Ellison moved to California in 1962, and subsequently began to sell his writing to Hollywood. He wrote the screenplay for The Oscar, 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.

Ellison alleged that James Cameron's film The Terminator drew from material from an episode of the original Outer Limits which Ellison had scripted, "Soldier" (1964). Hemdale, the production company and the distributor Orion Pictures, settled out of court for an undisclosed sum and added a credit to the film which acknowledged Ellison's work. Cameron objected to this acknowledgement and has since labeled Ellison's claim a "nuisance suit". Ellison has publicly referred to The Terminator as "a good film." Some accounts of the settlement state that another Outer Limits episode written by Ellison, "Demon with a Glass Hand" (also 1964), was also claimed to have been plagiarized by the film, but Ellison has explicitly stated that "'Terminator' was not stolen from 'Demon with a Glass Hand,' it was a ripoff of my OTHER Outer Limits script, 'Soldier.'"

Ellison continued to publish short fiction and nonfiction pieces in various publications, including some of his best known stories. "'Repent, Harlequin!' Said the Ticktockman" (1965) is a celebration of civil disobedience against repressive authority. "I Have No Mouth, and I Must Scream" (1967) is an allegory of Hell, where five humans are tormented by an all-knowing computer throughout eternity; the story was the basis of a 1995 computer game; Ellison participated in the game's design and provided the voice of the god-computer AM. Another story, "A Boy and His Dog", examines the nature of friendship and love in a violent, post-apocalyptic world and was made into the 1975 film of the same name, starring Don Johnson.[1]

In 1966, Ellison married his third wife, Lory Patrick. The marriage lasted only seven weeks.

Ellison has repeatedly criticized how Star Trek creator and Producer Gene Roddenberry (and others) rewrote his original script for the 1967 episode "The City on the Edge of Forever". Despite his objections, Ellison kept his own name on the shooting script instead of using "Cordwainer Bird" to indicate displeasure (above).

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) and Aggiecon XXXI (2000).

In the 1970s, Artist and Cartoonist Gordon Carleton wrote and drew a scripted slide show called "City on the Edge of Whatever", which was a spoof of "The City on the Edge of Forever". Occasionally performed at Star Trek conventions, it features an irate Writer named "Arlan Hellison" who screamed at his producers, "Art defilers! Script assassins!"

In the 1971 novel by Larry Niven and David Gerrold, The Flying Sorcerers, the names and characteristics of the local gods are playful derivatives of those of many science fiction authors and editors. The Ellison reference is "Elcin", the tiny god of thunder and lightning—alluding to Ellison's short stature and stormy personality.

The Last Dangerous Visions (TLDV), the third volume of Ellison's anthology series, was originally announced for publication in 1973 but remains unpublished. 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.

Ben Bova's 1975 novel The Starcrossed, a roman à clef about Bova and Ellison's experience on The Starlost TV series, features a character "Ron Gabriel" who is a pastiche of Ellison. Bova's novel is dedicated to Ellison's pseudonym "Cordwainer Bird", who was credited as series creator on The Starlost per Ellison's demand. In the novel, "Ron Gabriel" requires the fictional series producers to credit him under the pseudonym "Victor Lawrence Talbot Frankenstein".

In 1976, he married his fourth wife, Lori Horowitz. He was 41 and she was 19. He 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.

Shortly after the release of Star Wars (1977), Ben Roberts contacted Ellison to develop a script based on Isaac Asimov's I, Robot short story collection for Warner Brothers. In a meeting with the Head of Production at Warners, Robert Shapiro, Ellison concluded that Shapiro was commenting on the script without having read it and accused him of having the "intellectual and cranial capacity of an artichoke". Shortly afterwards, Ellison was dropped from the project. Without Ellison, the film came to a dead end, because subsequent scripts were unsatisfactory to potential Directors. After a change in studio heads, Warner allowed Ellison's script to be serialized in Isaac Asimov's Science Fiction Magazine and published in book form. The 2004 film I, Robot, starring Will Smith, has no connection to Ellison's script.

Ellison served as creative consultant to the 1980s version of The Twilight Zone science fiction TV series and Babylon 5. As a member of the Screen Actors Guild (SAG), he has voiceover credits for shows including The Pirates of Dark Water, Mother Goose and Grimm, Space Cases, Phantom 2040, and Babylon 5, as well as made an onscreen appearance in the Babylon 5 episode "The Face of the Enemy".

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

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 has often publicly boasted about the incident.

Ellison lives in Los Angeles with Susan, his fifth wife, whom he married in 1986. In 1994, he suffered a heart attack and was hospitalized for quadruple coronary artery bypass surgery. Since 2010 ("the worst, the lowest point in [his] life"), he also suffers from, and is under treatment for, clinical depression.

He was awarded the Silver Pen for Journalism by International PEN, the international writers' union. 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.

Ellison's 1992 short story "The Man Who Rowed Christopher Columbus Ashore" 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. 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".

Since the publication of the author's last collection of previously uncollected stories, Slippage (1997), Ellison has published the following works of fiction:

In March 1998, the National Women's Committee of Brandeis University honored him with their 1998 Words, Wit, Wisdom award. In 1990, Ellison was honored by International PEN for continuing commitment to artistic freedom and the battle against censorship.

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, internet Service providers 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 has 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, 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.

On Thursday, April 19, 2007, Dreams with Sharp Teeth received its first public screening at the Writers Guild Theatre in Los Angeles. The documentary about Ellison and his work was written and directed by Erik Nelson with archival footage of Ellison. It was released on DVD by New Video Group on May 26, 2009.

On the May 30, 2008 broadcast of the PRI radio program Studio 360, Ellison announced that he had signed with a "major publisher" to produce his memoirs. The tentative title is Working Without a Net. That title first appears in the television show Babylon 5, for which Ellison was a creative consultant. In an episode titled "TKO", the fictional character Susan Ivanova is once seen in year 2258 reading, and laughing to, a book titled Working Without A Net, by Harlan Ellison.

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.

Ellison is featured as a recurring character in the 2010 television show Scooby-Doo! Mystery Incorporated, playing a fictionalized version of himself as a professor of literature at the fictional Miskatonic University and colleague of H.P Hatecraft. His writings in speculative fiction give him a unique understanding of alternate timelines and place him in the role of an advisor to the protagonists. The character is voice-acted by Harlan Ellison himself and is reminiscent of Ellison's real-life personality.

In November 2011, Edgeworks Abbey (Ellison's personal publishing arm) and Spectrum Fantastic, published a pocket-sized gift book entitled Bugf#ck: The Useless Wit & Wisdom of Harlan Ellison. It contains quotes on writing, sex, politics, love and war, as well as pertinent excerpts from his short stories, and a handful of personal photographs of the author. In December 2011, Edgeworks Abbey began publishing original collections and retrospectives in two different series: the Brain Movies series (which contain teleplays from Ellison's award-winning career as a screenwriter) and the Harlan 101 series (which contain reprints, and original, unpublished stories and essays, and serve as an introduction to Ellison's writings). December 1, 2011 saw the simultaneous publication of four books: Brain Movies: Volume One, Brain Movies: Volume Two, Harlan 101: Encountering Ellison, and The Sound of a Scythe and Three Brilliant Novellas.

In November 2012, Edgeworks Abbey published None of the Above, an unproduced screenplay adaptation (written for Director Costa-Gavras) of Norman Spinrad's novel, Bug Jack Barron, and Rough Beasts, seventeen never-before-collected pulp stories from the 1950s.

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.

On about October 10, 2014, Ellison suffered a stroke. Although his speech and cognition are unimpaired, he suffered paralysis on his right side, for which he was expected to spend several weeks in physical therapy before being released from the hospital.

In December 2015, Subterranean Press published Can & Can'tankerous, containing previously uncollected short stories by Harlan Ellison.

Ellison has a reputation for being abrasive and argumentative. Ellison has generally agreed with this assessment, and a dust jacket from one of his books described him as "possibly the most contentious person on Earth". Ellison has filed numerous grievances and attempted lawsuits; as part of a dispute about fulfillment of a contract, he once sent 213 bricks to a publisher postage due, followed by a dead gopher via fourth-class mail. In an October 21, 2017 piece in Wired, Ellison was dubbed, "Sci-Fi's Most Controversial Figure."