Early in his career he was able to study the paintings of Frank Frazetta in the latter's studio. In the early 1970s he did a couple of cover paintings for Castle of Frankenstein magazine. Throughout the 1970s he was one of the foremost cover artists on Warren Publishing's Creepy and Eerie magazines.

He depicted Conan the Barbarian, Tarzan and the rock acts KISS, Manowar, Sleepy Hollow, Rainbow, and Ace Frehley.

His work often portrays exotic, enchanted locales and primal battlefields. He developed the artwork for Coheed and Cambria's album Good Apollo, I'm Burning Star IV, Volume Two: No World for Tomorrow, and a painting of his was used as the cover art for Alabama Thunderpussy's 2007 release, Open Fire. In 2012, one of Kelly's paintings was used for the cover of Electric Magma's 12" vinyl release Canadian Samurai II.

Kelly was a guest at the Kiss by Monster Mini Golf course in Las Vegas, Nevada, doing autograph signings of prints for the classic Kiss albums he drew cover artwork for.