The Encyclopedia of Science Fiction wrote: "More than any other individual, he helped to shape modern sf", and Darrell Schweitzer credits him with having "decreed that SF writers should pull themselves up out of the pulp mire and start writing intelligently, for adults." After 1950, new magazines such as Galaxy Science Fiction and The Magazine of Fantasy & Science Fiction moved in different directions and developed talented new writers who were not directly influenced by him. Campbell often suggested story ideas to writers (including "Write me a creature that thinks as well as a man, or better than a man, but not like a man"), and sometimes asked for stories to match cover paintings he had already bought.
Campbell had a strong formative influence on Asimov and eventually became a friend. Asimov said of Campbell's influence on the field:
By his own example and by his instruction and by his undeviating and persisting insistence, he forced first Astounding and then all science fiction into his mold. He abandoned the earlier orientation of the field. He demolished the stock characters who had filled it; eradicated the penny dreadful plots; extirpated the Sunday-supplement science. In a phrase, he blotted out the purple of pulp. Instead, he demanded that science-fiction writers understand science and understand people, a hard requirement that many of the established writers of the 1930s could not meet. Campbell did not compromise because of that: those who could not meet his requirements could not sell to him, and the carnage was as great as it had been in Hollywood a decade before, while silent movies had given way to the talkies.
One example of the type of speculative but plausible science fiction that Campbell demanded from his writers is "Deadline", a short story by Cleve Cartmill that appeared during the wartime year of 1944, a year before the detonation of the first atomic bomb. As Ben Bova, Campbell's successor as editor at Analog, wrote, it "described the basic facts of how to build an atomic bomb. Cartmill and Campbell worked together on the story, drawing their scientific information from papers published in the technical journals before the war. To them, the mechanics of constructing a uranium-fission bomb seemed perfectly obvious." The FBI descended on Campbell's office after the story appeared in print and demanded that the issue be removed from the newsstands. Campbell convinced them that by removing the magazine "the FBI would be advertising to everyone that such a project existed and was aimed at developing nuclear weapons" and the demand was dropped.
Campbell was also responsible for the grim and controversial ending of Tom Godwin's short story "The Cold Equations". Writer Joe Green recounted that Campbell had
three times sent 'Cold Equations' back to Godwin, before he got the version he wanted ... Godwin kept coming up with ingenious ways to save the girl! Since the strength of this deservedly classic story lies in the fact that the life of one young woman must be sacrificed to save the lives of many, it simply would not have the same impact if she had lived.
Between December 11, 1957, and June 13, 1958, Campbell hosted a weekly science fiction radio program called Exploring Tomorrow.