Winnie Johnson, 78, had gone through what she described as almost five decades of torture. Ian Brady and Myra Hindley, who were both serving life sentences for other killings, only admitted in 1987 that 12-year-old Keith Bennett was among their victims.
Johnson was in a hospice and died early Saturday morning, The Guardian reported.
Her death came days after reports surfaced in a Channel 4 documentary that Brady had given his mental health advocate, Jackie Powell, a letter to Johnson telling her where he buried Keith's body but had ordered her not to send it until after his death. Powell, 49, was detained for questioning and released on bail, and detectives searched her house without finding anything.
Johnson wrote Brady a letter in July begging for information before her death. She once said knowing her son might be lying alone forever filled her "every waking moment with dread."
"Ian Brady has tortured me for over 40 years and enjoyed every minute of it," she said. "He is like Hannibal Lecter, getting pleasure from mind games. He did his best to drive me mad, even though behind bars. I felt at times that his mind was in search of my soul. Many times he nearly pushed me over the edge."
Brady is being held in a secure hospital. Hindley died in 2002 in prison.
In 1987, the pair admitted they had killed Keith and 16-year-old Pauline Reade. Both were taken to Saddleworth Moor near Manchester but identified only Pauline's burial place.