Cornwell was born in Sharon, Pennsylvania to Sidney Cornwell and Robin Bodiford. However, he was primarily raised in Cleveland, Ohio and Louisville, Kentucky. At age 10, he relocated with his mother and stepfather Jackie Herring (died 2008) to Louisville where he first showed an interest in the arts in middle school. He graduated from Eastern High School where he participated in chorus and school plays. In addition, Justin was named as a premier student Artist at the Muhammad Ali Center where some of his artwork still hangs today. He attended the University of Louisville where he continued studying acting. Cornwell relocated to Chicago in 2011 to pursue his acting career.

Cornwell found work at the Chicago Shakespeare Theatre where he appeared in "Othello the Remix" and "A Mid Summers Night Dream" shortly after being signed by the Gray Talent Group. Justin booked appearances in commercials for Walmart and the Ohio Lottery. In 2014, Cornwell appeared in the direct-to-video movie, Alpha House, and in an episode of the NBC crime drama, Chicago P.D.. In 2015, he appeared in an episode of Fox's hit musical drama, Empire and later had an uncredited appearance in Spike Lee's Chi-Raq. In March 2016, Cornwell was cast in the pilot for the CBS drama Training Day, based on the 2001 film of the same name.