McMullen is one of Australia's leading science-fiction and fantasy authors and has written over 70 stories and 17 books. In 2011, his novelette "Eight Miles" was the runner-up in the Hugo Awards. He has won the Analog Reader's Award twice, for "Ninety Thousand Horses" in 2013 and "Tower of Wings" in 2002.
His first novel was originally published in Australia as two separate books, Voices in the Light (1994) and Mirrorsun Rising (1995). His first internationally published novel was The Centurion's Empire (1998), which featured a time machine built during the Roman Empire. After this book's success, his first two novels were rewritten and combined for a publication in the US as Souls in the Great Machine (1999), which, in turn, became the first volume of the Greatwinter trilogy, a unique mix of the generally anti-genres steampunk and cyberpunk. This was followed by the Moonworlds series, which saw McMullen blend science and romance in a fantasy setting. His most recent series is the Century War series for young adult readers, set in Melbourne in 1901.
McMullen's non-fiction work includes Strange Constellations: A History of Australian Science Fiction, a history of Australian science fiction jointly written with Van Ikin and Russell Blackford. He also co-wrote the first histories of Australian fantasy and horror with Steven Paulsen.
Far from Void: The History of Australian SF Magazines (1992) in Aurealis #7 (ed. Stephen Higgins, Dirk Strasser)
Skirting the Frontier (1992) in Eidolon Autumn 1992 (ed. Jeremy G. Byrne)
Showcase or Leading Edge: Australian SF Anthologies 1968–1990 (1992) in Aurealis #9, (ed. Stephen Higgins, Dirk Strasser)
From Science Fantasy to Galileo (1992) in Eidolon Spring 1992 (ed. Jeremy G. Byrne, Jonathan Strahan)
Australian Content: The State of Quarantine (1993) in Eidolon Summer 1993 (ed. Jeremy G. Byrne, Jonathan Strahan)
Australian Content: Suffering for Someone Else's Art (1993) in Eidolon Autumn 1993 (ed. Jonathan Strahan, Jeremy G. Byrne)
Protection, Liberation and the Cold, Dangerous Universe: The Great Australian SF Renaissance (1993) in Aurealis #11, (ed. Stephen Higgins, Dirk Strasser)
No Science Fiction Please, We're Australian (1993) in Eidolon Winter 1993 (ed. Jeremy G. Byrne, Jonathan Strahan)
The Quest for Australian Fantasy (1994, with Steven Paulsen) in Aurealis No. 13, (ed. Stephen Higgins, Dirk Strasser)
Australian Content: The Great Transition (1994) in Eidolon Winter 1994 (ed. Jonathan Strahan, Jeremy G. Byrne)
A History of Australian Horror (1995, with Bill Congreve and Steve Paulsen) in Bonescribes: Year's Best Australian Horror: 1995 (ed. Bill Congreve, Robert Hood)
1991 Best Australian Short Fiction – While the Gate is Open
1992 Best Short Fiction – Alone in His Chariot; William Atheling Jr. Award for Criticism – Going Commercial
1993 William Atheling Jr. Award for Criticism – Australian SF Art Turns 50
1996 Best Australian Long Fiction – Mirrorsun Rising; William Atheling Jr Award for Criticism – The Hunt for Australian Horror Fiction (together with Steven Paulsen and Bill Congreve)
1998 William Atheling Jr Award for Criticism – Fantasy in Australia (together with Steven Paulsen)
2000 William Atheling Jr Award for Criticism – Strange Constellations (together with Van Ikin and Russell Blackford)