O'Toole was born Annette Toole in Houston, Texas, the daughter of Dorothy Geraldine (née Niland) and William West Toole, Jr. Her mother taught dance, something O'Toole learned at the age of 3. She started taking acting lessons after family moved to Los Angeles when she was 13; her first role was on The Danny Kaye Show. She also had guest appearances in shows such as My Three Sons (1960), The Virginian, Gunsmoke, Hawaii Five-O, and The Partridge Family.