In 1993 Cassidy penned "Saturn: A Beginners Guide to Using the Internet" in response to a growing desire among the public to use the Internet. Cassidy followed up with Stickman's Way Cool Guide to Network Wizardry which used stick figure illustrations and humor to bring a technical subject to a popular audience.

Cassidy published two additional technology books, The Concise Guide to Enterprise Internetworking and Security. and Introduction to Windows 2000 Network Administration Cassidy also co-authored the paper "Can You Trust Your Email?" in 1993, warning of a flaw in the protocol used to deliver email which could allow information to be forged. The flaw wasn't addressed in time contributing to the rise of SPAM as a multimillion-dollar industry.

Cassidy is commonly attributed with inventing photo-blogging with his "Photo-a-Week" project which let viewers into his life on a weekly basis starting on January 1, 2000. With the advent of digital photography and blogging software, the project changed into more frequent photographs. As of 2008, there were approximately 20,000 daily viewers. A 2007 Washington Post article cited it as popular with their staff photographers.

His photographic subjects are known for flights of fancy and a distinct sense of humor. In them, laws of nature are applied inconsistently; people and objects are often levitating, non sequiturs, whimsey, and cryptozoological intrusions are common. "I think the world in my photographs is a lot darker in many ways than the real world that people insulate themselves in, but it's also a lot funnier. My world is malevolent but humorous, as opposed to the real world which is malevolent and relentless, but is often packaged in a friendly box and rabbit ears," he said in a 2004 interview with A.D. Amorosi in the October issue of Art Matters. His images often explore themes of "truth" and "fiction". This culminated in his July 2006 show "Lies" at the Sol gallery in Philadelphia. "Photography," he says in the artists statement for that show "is about lies just as much as it is about the truth."

When he lends his eye to serious subjects it takes on even more gravity. His work with cutters and homeless orphans presaged his 2004 fascination with American gun owners which lead to the critically acclaimed book, Armed America: Portraits of Gun Owners in Their Homes, which provided a unique view into the lives of a culture mysterious to some and ordinary to others. Praised by both advocates of gun control and gun ownership, the book made an exceptional splash and made Cassidy a photographer to watch. It was named one of the ten best art books of 2007 by Amazon.

Cassidy has been documenting American culture for more than two decades. He has photographed goths, punks, cutters, politicians, metalheads, dominatrices, authors and scholars in addition to less prosaic subjects. In recent years his projects have extended abroad to Romania, where he captured the lives of homeless orphans living in sewers; and to Egypt, where he reported on contemporary archaeological excavations.

His publications include several books on information technology, as well as a regular appearance as contributing editor for Videomaker Magazine. His work has appeared in the New York Times, Barron's Magazine, Photographers Forum, Asleep by Dawn, Gothic Beauty and numerous otherpublications. Amazon.com: Profile for Kyle Cassidy