The science fiction critic Gary Westfahl concluded a short biographical blurb in 2012: "despite having produced over 400 book covers, Nielsen has to date failed to inspire widespread admiration for his work."
Nielsen who has illustrated the covers of several children's books, including:
- Heir Apparent
- The Giver cover art.
- The Mortal Instruments series
- The Infernal Devices
- The Cirque Du Freak series
- The Chronicles of Narnia
- Ranger's Apprentice' series
- Tiger's Curse Series
Cliff Nielsen also illustrated the cover for the young adult novel Blood and Chocolate by Annette Curtis Klause and has done the covers for Amelia Atwater-Rhodes' Demon in My View, Midnight Predator and all the novels in her Kiesha'ra Series.
Besides this, he was known for working in the magazine Carpe Noctem, and was interviewed as well as created a few covers. He is currently designing/writing a graphic novel known as Beloved, but it so far, has only shown at ComicCon.
He also illustrated the cover of the Cirque du Freak Series.
Critic Joseph Szadkowski wrote that Nielsen's work for the card set Ultra X-Men: Wolverine makes him deserve "an honorary membership to the Der Blaue Reiter group". Nielsen has illustrated cards for the Magic: The Gathering collectible card game.
He has also done the covers for Cassandra Clare's young adult series The Mortal Instruments and The Infernal Devices published by Simon and Schuster.
- "Cliff Nielsen – Summary Bibliography". ISFDB. Retrieved 2014-12-20. Select a title to see its linked publication history and general information. Select a particular edition (title) for more data at that level, such as a front cover image or linked contents.
- "Nielsen, Cliff". Revised October 28, 2014. The Encyclopedia of Science Fiction (sf-encyclopedia.com). Retrieved 2014-12-20.
The entry is signed 'GW' for Gary Westfahl ("Contributors"). All information used here dates from the September 9, 2012, original version of the entry.
- "Cliff Nielsen". New York City: Shannon Associates (shannonassociates.com). Retrieved 2010-06-13.
- Felt, Kevin (November 17, 2007). "Painter of "Magic" cards inspiring random acts of kindness", San Gabriel Valley Tribune.
- Szadkowski, Joseph (November 16, 1996). "Wolverines in excelsis", The Washington Times, p. B4.
- Cliff Nielsen at agent Shannon Associates
- Cliff Nielsen at the Internet Speculative Fiction Database