Adam Rex grew up in Phoenix, Arizona, the middle of three children. He was neither the smart one (older brother) or the cute one (younger sister), but he was the one who could draw. He took a lot of art classes as a kid, trying to learn to draw better, and started painting when he was 11. Later he got a BFA from the University of Arizona, and met his physicist wife Marie. Adam and Marie live in Tucson, where Adam draws, paints and writes.

His first picture book, The Dirty Cowboy by Amy Timberlake, was published in 2003. His picture book Frankstein Makes a Sandwich, a collection of stories about monsters and their problems, was a New York Times Bestseller. 2007 saw the release of his first novel, The True Meaning Smekday. His most recent release, It Happened on a Train, is about seventh-grader Steve Brixton, who uncovers a mystery involving a fleet of priceless automobiles, an assassin, and a private rail car.