In 2004 he was nominated for an Eisner Award as "Best Cover Artist" for his work on Marvel's NYX, X-Men Unlimited, and New Mutants.

Middleton started his career as a comic book artist in 2000 as penciler for CrossGen's Meridian, written by Barbara Kesel.

[1] After contributing covers and short stories to Marvel's X-Men Unlimited series beginning in late 2002, he signed an exclusive contract in 2003,[2] and created NYX with Joe Quesada as writer, but left the title after four issues in 2004 when his contract ran out.

[1] Middleton worked as one of the conceptual artists on the 2005 science fiction film Serenity.

Middleton began working for WB Animation in October 2009 as a character designer, but was quickly promoted to be an art director for the Green Lantern: The Animated Series, where he has worked hand-in-hand with Bruce Timm to carve out the look and feel of the first Warner Brothers' CG television series.