Varney started his career in paper roleplaying games. From 1984 to 1985 he worked as Assistant Editor at Steve Jackson Games (with Warren Spector, then Editor-in-Chief) editing Space Gamer magazine.

In 1986, he left Steve Jackson Games to freelance. From this time onward, he wrote a large body of game supplements for companies like TSR, Inc., FASA Corporation, West End Games, and White Wolf, Inc..

Varney did work for TSR from 1987 to roughly 1992, including the HWA1-3 "Blood Brethren" trilogy (Nightwail, Nightrage, Nightstorm) and M4 Five Coins for a Kingdom, SJA1 Wildspace for Spelljammer, Veiled Alliance for Dark Sun, and several gamebooks, and the Ariya, Binsada, and Talinie realm packs for Birthright. He also edited several modules for the Ravenloft, Planescape, and Forgotten Realms settings, and was a game reviewer and news columnist for Dragon magazine.

In the early 2000s, he updated the classic 1980s roleplaying game Paranoia, published in 2004. He edited and packaged the game's support line for Mongoose Publishing until 2006.

The Austin e-learning company Enspire Learning produces a computer version of Varney's multiplayer business ethics and leadership simulation, the Executive Challenge. The Wall Street Journal covered the Executive Challenge in its May 10, 2004 issue.

Varney has long been involved in the game design and documentation for companies such as Origin Systems, Interplay, Prodigy, Acclaim Entertainment, Looking Glass Technologies, Microprose, and Sony Online Entertainment.

In 1993, Varney designed an expansion set for the trading card game The Gathering. This was not published, but the design concepts later surfaced in the web-based Vanguard format of the game, with Varney credited for the original concept.