Born in Los Angeles, California, the Dumont family lived in Irvine, California. Tom was the only adopted child in his family and has two siblings. Dumont's father, who played the piano, gave his son a guitar at age twelve. Dumont practiced by strumming folk songs by the likes of James Taylor. Dumont was influenced by heavy metal bands such as Iron Maiden, Judas Priest and KISS. He joined his older sister's heavy metal band Rising, but left in 1988 for third wave ska band No Doubt. He has one sister named Gina and a brother named John.