Born in Towanda, Illinois, Kath's parents were Arthur (1899–1977) and Eva Kath (1905–2000). She had a brother, Arthur (1926–2003), and a sister, Eleanor McConkey. "I was named after my grandmother Mary, and my grandfather Joe; and my mother added the -an onto the end of it. In spite of the spelling the family pronounced it as Jo Ann," Bonnie Lou noted in a 2007 interview. When the family home in Towanda burned down, they moved to Carlock, Illinois, where her father became a tenant farmer.