He won Nova Award for Best Fan Writer in 1991, the Pilgrim Award in 2002. He was elected to the First Fandom Hall of Fame in 2008. He received the Sam Moskowitz Archive Award in 2014. He is a member of The Esoteric Order of Dagon.
He edits the long-running Mammoth Book series of short story anthologies, each arranged around a particular theme in SF, fantasy, or mystery fiction. He has a special interest in fiction magazines and has written the multi-volume History of the Science Fiction Magazine.
With Marshall B. Tymn, he edited Science Fiction, Fantasy, and Weird Fiction Magazines (1985). He also assisted Frank H. Parnell in writing Monthly Terrors: An Index to the Weird Fantasy Magazines Published in the United States and Great Britain (1985). With Terry Jeeves we compiled The Complete Index to Astounding-Analog.
Ashley has contributed to many other genre reference works, including The Encyclopedia of Fantasy and The Encyclopedia of Science Fiction. He wrote the books to accompany the British Library's exhibitions, Taking Liberties (2008) and Out of This World (2011).