Ernst was born on July 28, 1922, in Jaffrey, New Hampshire. She had two younger brothers. She attended school in Europe and according to her, "I'd probably been better off in a school right here at home." Her family did not approve of her becoming an actress, enrolling in a drama school, and joining the Mercury Theater Group. Ernst did multiple jobs including nailing scenery and delivering posters by driving the company's Ford Model T.