A contemporary and friend of H. P. Lovecraft (Lovecraft's references to "le Comte d'Erlette" is an homage to Derleth), after Lovecraft's death he took a number of Lovecraft's unfinished stories and rewrote or finished them for publication in Weird Tales, and later in book form. In the process, he invented the term Cthulhu Mythos to describe the invented mythology that seemed to lay behind much of Lovecraft's fiction. Derleth codified the Mythos bringing it more in line with his own Christian conception of the battle between Good and Evil and, as other authors had done before him, he added new gods and creatures to the mythology.
Derleth founded Arkham House, a weird fiction specialty publisher.
He also wrote several novels about a British detective named "Solar Pons", quite similar in many respects to Sherlock Holmes. To British eyes, the novels have a curious quality due to Derleth's never having been anywhere near London when he wrote them.