Munn was a major early contributor to the pulp magazine Weird Tales during the 1920s and 1930s, with the editorship of Farnsworth Wright. A resurgence of interest in his work occurred during the 1970s due to its appearance in the Ballantine Adult Fantasy series and the successor fantasy series published with the imprint of Del Rey Books.

The two series of works for which he is known best, his Merlin saga and the Tales of the Werewolf Clan, were both started during the Weird Tales period. King of the World’s Edge, the first Merlin novel, was written as early as 1925. On publication it was compared favorably to the stories of Robert E. Howard, of whose fiction he confessed to being a great admirer. The first werewolf stories were written at the encouragement of H. P. Lovecraft. Both series were ended by the change of editors of the magazine from Farnsworth Wright to Dorothy McIlwraith; McIlwraith used different writers, eliminating the market for Munn’s work.

After Weird Tales ceased publishing his work, Munn generally did not seek new outlets, and his post-Weird Tales output was minor, most of it either self-published in small press editions or issued haphazardly by publishers who sought him. While he had already completed The Ship from Atlantis, the second installment of the Merlin Saga, it was published only years later, when Donald A. Wollheim contracted to publish King of the World's Edge in book form and also accepted the sequel.

The publication of his last great work of fantasy, Merlin's Ring, was also the result of a publisher seeking him. Reprising Wollheim’s role, Lin Carter, editor of the Ballantine Adult Fantasy series, learned of it while enquiring about the availability of the first two Merlin books. In the event, it was issued by Ballantine Books soon after the end of Carter’s connection with the publisher, in the interregnum between the Adult Fantasy series and Ballantine’s new Del Rey Books fantasy series. Del Rey later completed Carter’s original intention by reissuing both of the first two books in a single volume with the title of Merlin's Godson.

Similarly, Robert E. Weinberg was responsible for the revival and completion of the Werewolf Clan stories when he expressed an interest in reprinting them in his periodical Lost Fantasies. Munn had originally written eight werewolf stories for Weird Tales before its change of editorship; he now wrote two more to fill gaps in the sequence, and the entire series appeared in three parts in Lost Fantasies, nos. 4-6, 1976—77, as "Ten Tales of the Werewolf Clan." Afterward Munn wrote and self-published three additional stories to finish the series. The complete series was issued by Donald M. Grant, Publisher, Inc. as Tales of the Werewolf Clan, Volumes 1-2 (1979—80).

Some of Munn’s late horror stories were published in anthology series like Best Horror Stories of the year and Daw Books’ The Year's Best Horror Stories.

Merlin’s Ring brought Munn some long-delayed but limited recognition, being nominated for the 1975 Mythopoeic Fantasy Award and World Fantasy Award for Best Novel. Munn himself was nominated for the World Fantasy Award for Life Achievement during 1977, 1979, and 1980, and the Balrog Award for Professional Achievement during 1981. His poetry collection The Book of Munn was nominated for the 1980 Balrog Award for the categories of Collection/Anthology and Professional Publication, and his last novel, The Lost Legion, was also nominated for the 1981 Balrog Award for Novel. In every instance, however, Munn and his works failed to attain the award. He was, however, guest of honor of the 1978 World Fantasy Convention, and did win the Balrog Award for Poet during both 1980 and 1981.

Series

Merlin's Godson

• King of the World's Edge (1939, 1966)
• The Ship from Atlantis (1967)
• Merlin's Ring (1974)

The first two novels were also issued together as Merlin's Godson (1976). A projected final volume of the series to be named The Sword of Merlin was never finished.

Tales of the Werewolf Clan

Listing as originally published (incomplete; the titles of two early tales and two late ones have not yet been identified - see discussion page).

• "The Werewolf of Ponkert" (1925)
• "The Return of the Master" (1927)
• "The Werewolf's Daughter" (1928)
• "The Master Strikes" (1930)
• "The Master Fights" (1930)
• "The Master Has a Narrow Escape" (1931)
• In the Hulks: A Lost Tale of the Werewolf Clan (1979 - chap.)
• The Transient: A Lost Tale of the Werewolf Clan (1979 - chap.)
• In Regard to the Opening of Doors: A Lost Tale of the Werewolf Clan (1979 - chap.)

"The Werewolf of Ponkert" and "The Werewolf's Daughter" were issued together as:

• The Werewolf of Ponkert (1958)

All the tales were collected (and most of them retitled) in the volumes:

• Tales of the Werewolf Clan, Volume 1, In the Tomb of the Bishop (1979)

(contains "The Cat-Organ," "Hau! Hau! Huguenots," "The Wreck of the Santa Ysabel," "The Bug-Wolves of Castle Manglana," "In The Tomb of the Bishop," "The Leather Cannon," "Achsah Young...of Windsor")

• Tales of the Werewolf Clan, Volume 2, The Master Goes Home (1980)

(contains "The Master Meets A Worthy Foe," "The Diary," "In the Hulks," "In Regard to the Opening of Doors," "The Transients," "The Master Goes Home")

Historical novels

• The Lost Legion (1980)

Short stories

• "The City of Spiders" (1926)
• "The Chain" (1928)
• "A Sprig of Rosemary" (1933)
• "The Wheel" (1933)
• "Dreams May Come" (1939)
• Christmas Comes to the Little House (1974 - chap.)
• "The Black Captain" (1975)
• The Affair of the Cuckolded Warlock (1975 - chap.)
• "The Merlin Stone" (1977)
• "The Well" (1977)
• "The Stairway to the Sea" (1978)
• What Dreams May Come (1978 - chap.)
• "The De Pertriche Ring" (1979)
• The Baby Dryad: A Fanciful Christmas Tale (1980 - chap.)
• "The Wanderers of the Waters" (1981)

Poetry Collections

• The Banner of Joan (1975) (epic poem)
• Twenty Five Poems (1975)
• Seasons Greetings with Spooky Stuff (1976 - chap.)
• To All Amis (1976 - chap.)
• There was a Man (1977)
• The Pioneers (Part One) (1977)
• Dawn Woman (1979 - chap.)
• Fairy Gold (1979 - chap.)
• On Life and Love and Loneliness (1979 - chap.)
• The Book of Munn: or "a recipe for roast camel" (1979)

Poems

• "Cradle Song for an Abandoned Werewolf" (1973)
• "Fairy Gold" (1976)
• "Dante Returns from the City" (1976)
• "Limbo" (1976)

Nonfiction

• "HPL: a reminiscence" (1979)