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. He auditioned with the song "Ain't No Sunshine". He was one of only 13 people from Chicago, the lowest number out of all 7 audition cities, to make it through to Hollywood week. During Hollywood week his group included Aaron Kelly and Crystal Bowersox, all of whom made the Top 5. Crystal Bowersox and DeWyze made it to that top two, in which he won.

His closest friend among the competitors is Andrew Garcia.Garcia was his roommate throughout the season until he was eliminated in the Top 9. Lee stated that he was generally close to all the Top 10 Idols, but he is also especially close to Siobhan Magnus, Katie Stevens, Michael Lynche, Aaron Kelly, and Alex Lambert and Lacey Brown from Top 16. Before every performance, DeWyze would call his family.

DeWyze was one of two contestants in Season 9 to never land in the bottom three, the other being Crystal Bowersox. The judges praised him for shedding his shy persona throughout the season and showing, with confidence, that he was "in it to win it."

On May 14, 2010, DeWyze performed at the Arlington Park Race Track in Arlington Heights, Illinois, with more than 41,000 people attending as part of his American Idol Homecoming day. Along with performing at the race track he had a parade in Mount Prospect, performed at a local AT&T store, and threw out the opening pitch at a Chicago Cubs baseball game.

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