His literary criticism focuses upon the worldviews of authors. His The Weird Tale looks at horror and fantasy writing by Arthur Machen, Algernon Blackwood, Dunsany, M. R. James, Bierce and Lovecraft.
The Modern Weird Tale examines modern writers, including Shirley Jackson, Ramsey Campbell, Stephen King, Robert Aickman, Thomas Ligotti, T. E. D. Klein and others, from a similar viewpoint. The The Evolution of the Weird Tale (2004), includes essays on Dennis Etchison, L. P. Hartley, Les Daniels, E. F. Benson, Rudyard Kipling, David J. Schow, Robert Bloch, L. P. Davies, Edward Lucas White, Rod Serling, Poppy Z. Brite and others. Unutterable Horror: A History of Supernatural Fiction (published in two volumes, 2012 by PS Publishing) is a history of supernatural fiction from Gilgamesh.
Joshi and David E. Schultz edited volumes of Lovecraft's letters with Necronomicon Press (including those to Robert Bloch, Henry Kuttner, Samuel Loveman and Vincent Starrett); for Night Shade Books (Mysteries of Time and Spirit: Letters to Donald Wandrei) and Letters from New York; and for University of Tampa Press (O Fortunate Floridian: Letters to Robert H. Barlow).
Joshi and Schultz are now progressively issuing volumes through Hippocampus Press H. P. Lovecraft's letters to correspondents. Lovecraft's letters to Rheinhart Kleiner; Alfred Galpin; August Derleth (2 volumes); Robert E. Howard (2 volumes); James F. Morton; Elizabeth Toldridge & Anne Tillery Renshaw; F. Lee Baldwin,
Duane Rimel & Nils Frome; J. Vernon Shea, Carl F. Strauch & Lee McBride White; Robert Bloch; and Clark Ashton Smith have been published.
Joshi edited the five-volume set of Lovecraft's Collected Essays issued by Hippocampus Press from 2004-2006. He edited two annotated volumes of Lovecraft's best work for Dell books (the second with Peter H. Cannon). He and David E. Schultz edited the collected poetry of Clark Ashton Smith, issued by Hippocampus Press (3 volumes, 2007–2008) and the collected poetry of George Sterling (3 volumes, 2013).
Joshi was general editor of Cthulhu Mythos works from Perilous Press, including works by Michael Shea and Brian Stableford. The first publication was Shea's Copping Squid and Other Mythos Tales (2009), with Stableford's volume titled The Womb of Time (two Mythos novellas).
Joshi has published on atheism, including Documents of American Prejudice (1999), an annotated collection of American racist writings; In Her Place (2006), which collects written examples of prejudice against women; and Atheism: A Reader (2000), which collects atheistic writings by Antony Flew, George Eliot, Bertrand Russell, Emma Goldman, Gore Vidal and Carl Sagan, among others. An Agnostic Reader, collecting pieces by Isaac Asimov, John William Draper, Albert Einstein, Frederic Harrison, Thomas Henry Huxley, Robert Ingersoll, Corliss Lamont, Arthur Schopenhauer and Edward Westermarck, was published in 2007.
Joshi published God's Defenders: What They Believe and Why They Are Wrong (2003), an anti-religious polemic against various writers including C. S. Lewis, G.K. Chesterton, T.S. Eliot, William F. Buckley, Jr., William James, Stephen L. Carter, Annie Dillard, Reynolds Price, Elisabeth Kübler-Ross, Guenter Lewy, Neale Donald Walsch and Jerry Falwell. It is dedicated to theologian Robert M. Price.
In 2006 he published The Angry Right: Why Conservatives Keep Getting It Wrong, which criticized the political writings of such commentators as William F. Buckley, Jr., Russell Kirk, David and Rush Limbaugh, Ann Coulter, Phyllis Schlafly, William Bennett, Gertrude Himmelfarb, Irving Kristol and William Kristol, arguing that Americans have become steadily more liberal.
In 1987, Joshi became the fifth Official Editor of the Esoteric Order of Dagon.
Joshi has edited Lovecraft Studies (1979–2001) and Studies in Weird Fiction (1986–2005), both published by Necronomicon Press; and Studies in the Fantastic (2008–09), published by the University of Tampa Press. He is editor of Weird Fiction Review (Centipede Press; 2010), and the Hippocampus Press journals Spectral Realms (from 2014); Lovecraft Annual (from 2007); Nemesis (from 2016). He was formerly co-editor of Dead Reckonings (from 2007).
In 2011, Joshi became editor-in-chief of The American Rationalist magazine.