[2] Leon first began working professionally at the age of 16, with a series of black and white illustrations for TSR's Dragon and Dungeon magazines.

[3] He majored in illustration at New York's School of Visual Arts, studying under artists such as Will Eisner, Walter Simonson, and Jack Potter.

[8] Regarding this milestone, Leon would relate to Newsarama in 2019:[9] "When I graduated SVA, I remember they had us fill out a form, for what purpose, I don't recall.

[5] By the early 2000s, Leon's work caught the notice of DC Comics' parent company Warner Bros.,[5] through which Leon would provid artwork for a number of style guides for the studio's feature film adaptations of DC properties, including Superman Returns, Batman Begins, Green Lantern, and The Dark Knight.

[9] In April 2021, Leon was announced as the artist on the upcoming Batman/Catwoman Special #1, a retelling of the origin of Catwoman, written by Tom King.

[15] Marvel's featured a eulogy by Marvel Entertainment EVP Creative Director Joe Quesada, who stated, "John Paul Leon was truly a unique talent, admired and envied by all of us in the industry for his ability to convey the complexities of the world with a distilled simplicity of line and mastery of the craft that few can ever hope to achieve.

"[16] In DC's books, Publisher and Chief Creative Officer Jim Lee wrote, "One of the greatest artists of our generation, he was also one of the nicest and most talented creators one could be lucky enough to have met."