Dun was born in Columbus, Ohio, to Laura Lee Dun (née McCollum), a hospice social worker, and William Earl "Bill" Dun, a physical therapy assistant. He grew up with two sisters, Ashley Bonnie Lee and Abigail Christine "Abbie" Dun, and a brother named Jordan Christopher Dun. Dun states that his adolescence was somewhat rebellious. "I just had this aggression," he says, noting that his parents nearly kicked him out when he was 14. "They almost sent me to military school. They didn't know what to do with me, and I was always in detention. I never got into drugs or alcohol, but I would yell at my parents and just treat them terribly. Everything was an argument."
Dun did not attend college, instead living with friends and focusing on musical hobbies. "I was going nowhere," he says. "One day I said to my dad, 'Are you disappointed that I'm working a minimum-wage job, and I didn't go to college?' I'll never forget his response. He replied, 'It's not about how much money you make or what your job is, but it's more about your character. For that, I'm proud of you.' It gave me motivation."[7] Dun worked at Guitar Center for three years alongside his friends. It was here he met former Twenty One Pilots drummer Chris Salih, who introduced him to future bandmate Tyler Joseph.