Clements speaks both Chinese and Japanese, and many of his works relate to East Asia. He wrote his 1995 Master's degree at the University of Stirling on manga and anime exports, predicting the rise of several trends in the international industry including back-to-front printing, direct American investment in anime, and the proliferation of attempts to substitute non-Japanese products. Subsequently he translated over 70 anime and manga works for British distributors, and worked as a voice director and actor.

He served two years at Titan Books in London as the editor of Manga Max magazine, an experience he would later fictionalise as the Judge Dredd adventure Trapped on Titan. In 2000, he received the Japan Festival Award for Outstanding Contribution to the Understanding of Japanese Culture, specifically for his work on Manga Max magazine.

He left to work as a scriptwriter on an abortive Astro Boy movie project, and wrote several episodes of Halcyon Sun for Kuju Entertainment. He also served as a writer or translator on TV programs, including Japanorama (BBC Three), Saiko Exciting (Sci-Fi Channel) and Godzilla: King of the Monsters (BBC Two). Much of his fiction work is CD audio drama or radio under the auspices of Big Finish Productions, including the Strontium Dog series, starring Simon Pegg, and the Doctor Who spin-off Sympathy for the Devil, starring David Warner and David Tennant.

• Down to Earth (London: Big Finish, 2002; web, BBCi, 2005) (Strontium Dog)
• Trapped on Titan (London: Big Finish, 2002) (Judge Dredd)
• Unbound: Sympathy for the Devil (London: Big Finish, 2003) (Doctor Who)
• Fire From Heaven (London: Big Finish, 2003 web, BBCi, 2005) (Strontium Dog)
• 99 Code Red (London: Big Finish, 2003) (Judge Dredd)
• Pre-Emptive Revenge (London: Big Finish, 2004) (Judge Dredd)
• Solo (London: Big Finish, 2005) (Judge Dredd)
• Snake Head (London: Big Finish, 2005) (Doctor Who: UNIT)
• Red Devils (London: Noise Monster, 2005) (Space 1889)
• Immortal Beloved (London: Big Finish, 2007; radio, BBC7, 2008) (Doctor Who)
• Brave New Town (London: Big Finish, 2008; radio, BBC7, 2009) (Doctor Who)
• The Destroyer of Delights (London: Big Finish, 2009) (Doctor Who)
• The Tiger's Tail (London: Big Finish, 2009) (Robin Hood)
• The Deer Hunters (London: Big Finish, 2009) (Robin Hood)
• Secret of the Sword (London: Big Finish, 2009) (Highlander)
• Survival of the Fittest (London: Big Finish, 2010) (Doctor Who)
• The Devil's Playground (London: Big Finish, 2010) (Judge Dredd)

Other work includes the script for the comic Tastes Like Chicken in the Judge Dredd Megazine, as well as assorted short stories both there and in Doctor Who anthologies. His most famous work, Schoolgirl Milky Crisis, was the name for a fictional TV series that Clements often used in his Newtype USA columns in order to avoid breaking various non-disclosure agreements regarding real titles that he had worked on as a writer, director or translator. The name was later used as the title to a collection of Clements's articles and speeches.

Clements is a prolific author, whose many non-fiction publications, on subjects ranging from the history of the Vikings to the life of Chairman Mao, serve as research for his fiction. His books have been translated into a dozen languages, including Spanish, Portuguese, Dutch, and Korean. His major works include:

• The Moon in the Pines (2000, reprinted in paperback as Zen Haiku, 2007)
• A Guide to Japanese Animation Since 1917 (1st ed. 2001, 2nd ed. 2006, with Helen McCarthy)
• The Dorama Encyclopedia: A Guide to Japanese TV Drama Since 1953 (2003, with Motoko Tamamuro)
• The Pirate King: Coxinga and the Fall of the Ming Dynasty (2004, publ. in paperback as Coxinga, 2005)
• Confucius: A Biography (2004)
• A Brief History of the Vikings (2005)
• The First Emperor of China (2006)
• Mao (2006)
• Wu (2007)
• Marco Polo (2007)
• Beijing: The Biography of a City (2008)
• Makers of the Modern World: Prince Saionji (2008)
• Makers of the Modern World: Wellington Koo (2008)
• Schoolgirl Milky Crisis: Adventures in the Anime and Manga Trade (2009)
• President, Soldier, Spy (2009)
• A Brief History of the Samurai (2010)
• Nelson of the East (2010)