Middleton started his career as a comic book artist in 2000 as a penciler for CrossGen's Meridian, written by Barbara Kesel. After that, he joined a small independent British publisher, Com.x, planning to release there his creator-owned series Sky Between Branches; however, he only did a preview issue in early 2002, along with some other work for the publisher. Middleton began working for WB Animation in October 2009 as a character designer but was quickly promoted to be an art director for Green Lantern: The Animated Series.