DeWyze was born in Mount Prospect, Illinois, the eldest son of Kathleen (née Donahue) and Leon (Lee) DeWyze, Sr., who worked as a postal carrier in Elk Grove Village, Illinois. He has two older sisters, Shannon and Sarah, and a younger brother, Michael. His musical influences include Cat Stevens, Eric Clapton, Ben Harper, and Simon & Garfunkel.
After expulsion from high school, DeWyze worked as a paint salesman. He also worked as a trading floor clerk at the Chicago Mercantile Exchange in 2007–2008 before returning to work at the Mount Prospect paint store until he auditioned for American Idol and making it through to the Hollywood rounds of the show. The store continued to support him during his American Idol run by selling tee shirts encouraging fans to vote for Lee.
DeWyze auditioned in Chicago at the United Center for the ninth season of American Idol on June 22, 2009, after encouragement from his friend Vincent Ferrara who was also trying out for the contest in Chicago.