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.