Harrison was an extremely popular figure in the SF world, renowned for being amiable, outspoken and endlessly amusing. His quickfire, machine-gun delivery of words was a delight to hear, and a reward to unravel: he was funny and self-aware, he enjoyed reporting the follies of others, he distrusted generals, prime ministers and tax officials with sardonic and cruel wit, and above all he made plain his acute intelligence and astonishing range of moral, ethical and literary sensibilities.
Harrison's novelette "Down to Earth" took the cover of the November 1963 issue of Amazing Stories
Before becoming an editor and writer, Harrison started in the science fiction field as an illustrator, notably with EC Comics' two science fiction comic book series, Weird Fantasy and Weird Science. In these and other comic book stories, he most often worked with Wally Wood. Wood usually inked over Harrison's layouts, and the two freelanced for several publishers and genres, including westerns and horror comics. He and Wood split up their partnership in 1950 and went their separate ways. Harrison used house pen names such as Wade Kaempfert and Philip St. John to edit magazines and published other fiction under the pen names Felix Boyd and Hank Dempsey (see Personal Life below). Harrison ghostwroteVendetta for the Saint, one of the long-running series of novels featuring Leslie Charteris' character, The Saint. Harrison also wrote for syndicated comic strips, writing several stories for the character Rick Random.
His most popular and best-known work is contained in fast-moving parodies, homages or even straight reconstructions of traditional space-opera adventures. He wrote several named series of these: notably the Deathworld series (three titles, starting in 1960), the Stainless Steel Rat books (12 titles, from 1961), and the sequence of books about Bill, the Galactic Hero (seven titles, from 1965). These books all present interesting contradictions: while being exactly what they might superficially seem to be, unpretentious action novels with a strong streak of humour, they are also satirical, knowing, subversive, unapologetically anti-military, anti-authority and anti-violence. Harrison wrote such novels in the idiom of the politically conservativehack writer, but in reality he had a liberal conscience and a sharp awareness of the lack of literary values in so much of the SF he was parodying.
Adi Robertson agreed: "His books toed the line between science fiction adventure, humor, and satire, often with a strong anti-military bent informed by his time in the US Army Air Corps."
During the 1950s and 1960s, he was the main writer of the Flash Gordon newspaper strip. One of his Flash Gordon scripts was serialized in Comics Revue magazine. Harrison drew sketches to help the artist be more scientifically accurate, which the artist largely ignored.
Not all of Harrison's writing was comic, though. He wrote many stories on serious themes, of which by far the best known is the novel about overpopulation and consumption of the world's resources, Make Room! Make Room! (1966), which was used as a basis for the 1973 science fiction film Soylent Green (though the film changed the plot and theme).
For a time Harrison was closely associated with Brian Aldiss. They collaborated on a series of anthology projects and did much in the 1970s to raise the standards of criticism in the field, including institution of the John W. Campbell Memorial Award for Best Science Fiction Novel. Priest wrote, "In 1965 Harrison and Aldiss published the first issue (of two) of the world's first serious journal of SF criticism, SF Horizons. Together they edited many anthologies of short stories, each one illustrating the major themes of SF, and although not intended as critical apparatus the books were a way of delineating the unique material of the fantastic. As committed internationalists, the two men created World SF, an organisation of professionals intended to encourage and enhance the writing of non-anglophone SF." In particular, the two edited nine volumes of The Year's Best Science Fiction anthology series as well as three volumes of the Decade series, collecting science fiction of the 1940s, 1950s and 1960s respectively.