Ward created a student film titled Barrista starring Bueno the Bear, released in 2005 on Channel Frederator.

There he worked with several people who would later join him on the Adventure Time series, including composer Casey James Basichis, Adam Muto and Niki Yang, many of whom had attended the California Institute of the Arts alongside Pen.

[citation needed] In 2007, Ward was hired to work on the first season of Cartoon Network's The Marvelous Misadventures of Flapjack as a writer and storyboard artist.

Created by Thurop Van Orman, Flapjack was a storyboard-driven show which allowed the storyboard artists to write all the dialogue and draw all the action based on an outline assigned to them.

The artists each worked in pairs; Ward was partnered up with Mike Roth (who became the supervising producer of Regular Show) and later Alex Hirsch (who has since gone on to create Gravity Falls and develop Fish Hooks).

Pen credits his experience with Flapjack and working under Orman's "fun factory" philosophy had a significant impact upon his own writing process.

[13][14][15] Sometime during the fifth season of Adventure Time, Ward abruptly stepped down as showrunner, explaining it was negatively affecting his "quality of life".

However, he continued to work as one of the show's writers and storyboard artists until the end of season six, and still served as an executive producer up until the series finale.

[citation needed] In 2019, Card of Darkness, an Apple Arcade exclusive game, was released with Ward serving as art director.

[citation needed] On March 16, 2020, a trailer was released for the adult animated Netflix original show The Midnight Gospel,[18] co-created by Ward and comedian Duncan Trussell.