Dempsey was born in Lewiston, Maine, and grew up in the nearby towns of Turner and Buckfield. He has two older sisters and a half-brother, Shane. His mother, Amanda (née Casson), was a school secretary, and his father, William, was an insurance no: The youngest of three children, Dempsey was born in in the rural Maine town of Lewiston. His father, William, sold insurance and his mother, Amanda, worked as a secretary. The Dempsey family was Catholic. As such, Dempsey attended St. Dominic's Regional High School, as well as Buck-field High.