Cantwell was born in Chicago, Illinois. In the 1980s, the family moved to Dallas, Texas. Cantwell went to University of Southern California and enrolled in its screenwriting program as an undergraduate. He began writing with Chris Rogers in August of 2010, and they created Halt and Catch Fire together. He is one of the two creators of the TV series Halt and Catch Fire. Cantwell is directing the upcoming film The Parts You Lose.