Note: We have only included the publisher of the first edition of each book. Unless a subsequent edition was significantly revised, it would not be listed.

• The Enchanted Empire
  1959
  (Family Publications)

• The Dark King's Court
  1964
  (Jackdaw Books)

• Cruising the Mobius Strip
  1965
  (Jackdaw Books)

• Psi Alpha
  1965
  (Jackdaw Books)

• Galactic Odyssey
  1966
  (Family Publications)

• Superagent Z
  (as Lyle Hanske)
  1966
  (Family Publications)

• To Conquer Mankind
  1967
  (Jackdaw Books)

• Overman
  1968
  (Jackdaw Books)

• S'ldak and the Enchanted Empire
  1968
  (Jackdaw Books)

• S'ldak on the Fourth Moon
  1969
  (Jackdaw Books)

• Z Marks the Spot: A Superagent Z novel
  (as Lyle Hanske)
  1969
  (Family Publications)

• Brothers of the Sun
  1969
  (Anorak)

• Remains
  1971
  (Jackdaw Books)

• The Last Man on Mars
  1971
  (Jackdaw Books)

• Kundalini Eye, Metaphysics Detective
  1971
  (Anorak)

• Dan Hugo, Man of Stone: High Adventure #1
  (as Allen Shyke)
  1972
  (Family Publications)

• Zandar the Mystic: High Adventure #2
  (as Allen Shyke)
  1972
  (Family Publications)

• A Covenant with Fate
  1973
  (Anorak)

• The Raven: High Adventure #3
  (as Allen Shyke)
  1973
  (Family Publications)

• Captain Eclipse: High Adventure #4
  (as Allen Shyke)
  1973
  (Family Publications)

• The Justice Alliance: High Adventure #5
  (as Allen Shyke)
  1974
  (Family Publications)

• Johnny Weird
  1974
  (Jackdaw Books)

• The Sex Room and Other Stories
  1975
  (Anorak)

• Consequences of Deicide
  1975
  (Family Publications)

• Gift From a Blind God
  1976
  (Halloway)

• Bicentennial Blues: A Superagent Z Novel
  (as Lyle Hanske)
  1976
  (Family Publications)

• In Service to the Dark King's Court
  1977
  (Halloway)

• Against the Enchanted Empire
  1977
  (Halloway)

• The Taboo Land
  (as Allen Shyke)
  1977
  (Family Publications)

• The Gods of Infinity
  1978
  (Halloway)

• The Future is Past
  1978
  (Halloway)
  

• A Covenant With Fate
  1978
  (Revised Halloway edition)
  

• Cosmos Combat
  (as Lyle Hanske)
  1978
  (Family Publications)

• Plastic Orpheus
  1978
  (Anorak)

• If You Lived Here
  1979
  (Anorak)

• A Parade of Pleasure: Slouching Toward Oblivion
  1981
  (Halloway)

• The Sword and the Scroll
  1982
  (Jackdaw Books)

• Not Quite Dead
  (as Helen K. Slay)
  1983
  (ERB)

• Therein Lies Madness
  (as Helen K. Slay)
  1984
  (ERB)

• Language of Angels
  1985
  Graphic Novel, art by Dan Werd
  (Skulltrain Enterprises)

• The Gravediggers of 1933
  1986
  (as Helen K. Slay)
  (ERB)

• A Congress Of Shadows
  1986 (Jackdaw Books)

• Unholy Three: A Trio of Terror Tales
  (as Helen K. Slay)
  1987
  (ERB)

• Pirates of the Plains
  1988
  (Jackdaw Books)

• Man & Overman
  1989
  (Anorak)

• The Condemned One and Other Tales
  (as Helen K. Slay)
  1990
  (ERB)

• Future Fables
  1993
  (Fuse)

• Divine Intervention: A Tale of Terror
  1997
  (as Helen K. Slay)
  (ERB)

• Circles
  1998
  (DKC)

• Funnybooks
  1999
  (DKC)

• Replicons
  2000
  (Eyestory Press)

• Dimension Drug
  2001
  (Eyestory Press)

• The Saga Of S'ldak Omnibus Volume One
  2002
  (DKC)

• Best of Seamus McMahon
  2002
  (DKC)

• Universe 2.0
  2002
  (DKC)

• The Saga Of S'ldak Omnibus Volume Two
  2003
  (DKC)

• Tales From The Mythospheres
  2003
  (Blackblade)

• Echo Human
  2004
  (Blackblade)

• The Star Deviants
  2004
  (Eyestory Press)

• Fugue State
  2005
  (Blackblade)

• Nudie Cuties
  2005
  (DKC)