While still in college at Columbus College of Art and Design, Palencar exhibited work at the Society of Illustrators and the work on display attracted the attention of Byron Preiss. The result would be an early and prominent assignment for the artist in 1982 with the commission to illustrate The Secret, a puzzle book produced by Byron Preiss and published by Bantam Books. The illustrations for the book depict the visual components of 12 puzzles, each of which lead the reader on a treasure hunt. Preiss had hidden ceramic boxes, each redeemable for a jewel, in twelve cities and, to date, only three of the puzzles have been solved and the jewels recovered.
Palencar's first introduction to illustrating for fantasy and speculative fiction came shortly after college when working for Time-Life Books on The Enchanted World Series. The young artist would contribute to many books in the series, including Legends of Valor (1984), Ghosts (1984), and Night Creatures (1985).
His work has since appeared on hundreds of book covers for just about every major publisher in the U.S. and in over thirty countries around the world. The artist has created cover art for such authors as H. P. Lovecraft, Ursula Le Guin, Marion Zimmer Bradley, P.D. James, Charles de Lint, R.L. Stine, Octavia Butler, David Brin, and Stephen King. New York Times best-selling author Christopher Paolini, a fan of John's work, named the lead character's birthplace "Palancar Valley" after John in his NY Times Bestseller, Eragon for which John painted the cover as well as created the type treatment that would be used for the remaining three books in the series. Stephen King owns three works by the artist from the Dark Tower series.
Palencar has also created editorial illustrations for Time Magazine, Smithsonian Magazine, National Geographic, and Nat. Geo. Television as well as worked on entertainment projects for Lucas Arts, Paramount Pictures, and Vivendi Universal.
In 2000, Palencar became the Artist in Residence at the Cill Rialaig Arts Centre, in County Kerry, Ireland.
In 2007, Underwood Books published Origins:The Art of John Jude Palencar, a 128-page overview of the artist's work with a foreword by Christopher Paolini.