As a writer of science fiction, John W. Campbell Jr was hugely influential. As the Wikipedia page points out, he had his first story, "When The Atoms Failed", published in the January 1930 edition of Amazing Stories when he was still only 18. He soon became one of the "big names" of the time, writing space opera often featuring a trio of adventurers, Arcot, Wade & Morey (as seen in the The Black Star Passes and Islands of Space Ace editions) and the Penton & Blake duo (as seen in The Planeteers). But science fiction was in a time of transition then, and new names were appearing, such as Don A. Stuart, with a more sophisticated tone... his first story, "Twilight", in the November 1934 Astounding, immediately established him as a name to be reckoned with. Anyone who knew that Campbell's wife at the time was called Dona Stuart might have made an educated guess as to the identity of this new writer!
Astounding's August 1938 issue contained Stuart's "Who Goes There?", perhaps Campbell's last significant work of fiction - it was filmed as "The Thing From Another World" in 1951, and remade 31 years later as "The Thing", with its malevolent alien shape-shifter in an Antarctic base. But from late 1937, Campbell's energies went into editing Astounding, turning it from an sf pulp into, well, the science fiction magazine, and the home of science fiction's Golden Age. The May 1938 issue is the one with which he was given full charge, though he had been buying the stories and getting things ready since F. Orlin Tremaine had hired him the previous autumn. And there he stayed - also creating and editing Unknown (later Unknown Worlds), a brave attempt to do for fantasy what he was doing for sf. He wanted to get rid of the "Astounding" and just call the magazine Science Fiction, but the name has already been taken; for a while the "Astounding" part got awfully small on the cover, but finally, in 1960, the "Astounding" faded away, and the magazine became the more engineer-friendly Analog. And so it continues, though Campbell himself died in 1971.
The golden age of Astounding probably ended in around 1950, when Campbell was becoming sidetracked from the regular kind of sf more into "psi" stories and some not altogether mainstream new scientific discoveries, such as the "Dean Drive" and "Dianetics". As Isaac Asimov remarked, "Campbell championed far-out ideas.... He pained very many of the men he had trained (including me) in doing so, but felt it was his duty to stir up the minds of his readers and force curiosity right out to the border lines." That he certainly did, and while "ASF" might no longer have been the supreme market for science fiction, it still held its own against newer magazines. One thing is for sure, though, without Campbell's editorial talents, the world of science fiction would have been a much duller place.