The son of a cardiologist, Trohman was born in Hollywood, Florida. He was raised in South Russell, Ohio before his family moved to the Chicago area. His family is Jewish, although he noted to JVibe, "I think we were more Jewish culturally than we were religiously because after my brother's bar mitzvah, we stopped going to synagogue entirely." Trohman attended New Trier High School in Winnetka, Illinois, and played bass with fellow Fall Out Boy band member Pete Wentz in a band.