Hannan is the creator and executive producer of the Nickelodeon animated television series CatDog, overseeing all aspects (writing, storyboarding, character design, art direction, and post-production) of over one hundred 11-minute episodes and holiday specials. He also produced Fetch, the CatDog theatrical short subject and a ninety-minute television movie called CatDog and the Great Parent Mystery. He also wrote and sang the CatDog theme song.
Hannan is the author and illustrator of The Sillyville Saga: Sillyville or Bust, Escape from Camp Wannabarf, School After Dark: Lessons in Lunacy, and The Battle of Sillyville: Live Silly or Die! As of 2008 Hannan is working on a variety of film, television, game, and book projects, including a series of six middle-grade illustrated novels for HarperCollins called Super Goofballs, which features a staggering group of avenging lunatics: Super Goofballs #1: That Stinking Feeling, Super Goofballs #2: Goofballs in Paradise, Super Goofballs #3: Super Underwear..and Beyond!, Super Goofballs #4: Attack of the 50-Foot Alien Creep-oids!, Super Goofballs #5: Doomed in Dreamland, Super Goofballs #6: Battle of the Brain-sucking Robots.
As of 2008 he is working on a picture book and another series of middle-grade illustrated novels called Wally, King of Flurb — also for HarperCollins. He is working on a graphic novel and an animated television series.
Hannan’s writing, illustrations, and single-panel cartoons (The Adventures of a Huge Mouth) have appeared in lots of newspapers and magazines and he has exhibited his paintings and drawings. He created the web series Really Freaking Embarrassing for Shut Up! Cartoons.