Anthony Borrowdale smashed a bedroom window, climbed in and turned off the lights so the victim, who lives on her own, could not identify him.
As he raped her, the victim was crying and shouting for help but nobody heard her, Sheffield Crown Court heard.
The vulnerable pensioner, who suffers from Parkinson’s, kidney and heart disease also has hearing and speech impairments which makes it hard for her to communicate.
Just two days after the incident she wrote a note for a carer which read: “I can’t forget what that man has done to me.”
Borrowdale, 56, who has a wife and daughter, is believed to have gone off the rails after his marriage broke down.
The court heard he had been acting bizarrely in his neighbourhood, knocking on windows at passing schoolchildren, urinating out of his bedroom window, and burping out of his letter-box.
He was arrested for indecently exposing himself at schoolgirls at a bus stop 18 months ago but the case was dropped owing to lack of evidence.
Rachael Harrison, prosecuting, said the 89-year-old, who never went out of her council bungalow in Barnsley but often sat at the window, used a walking frame to get around. She had to write things down to communicate.
Three care workers visited daily and she was left at 6.15pm on May 31 this year in an armchair in her sitting room, wearing her nightie and dressing gown and happily watching television.
About 11.10pm a next-door neighbour heard a smashing sound outside and went to investigate but could find nothing amiss so went back to bed.
At midnight the distressed victim knocked on her neighbour’s door at the other side but she could not make him understand what was wrong.
Eventually she activated her personal alarm at 12.43am and call centre staff contacted the emergency services. When a paramedic arrived the police were called.
Miss Harrison said: “She wrote replies to questions and said ‘he raped me’.”
Police recovered her nightdress and forensic tests and fingerprint evidence led them to arrest Borrowdale two days later. He made no comment during interviews.
His victim was taken to a care home and she has never returned to her old bungalow. She was not fit to give evidence in court.
Borrowdale, of Copeland Road, Wombwell, near Barnsley admitted rape. He has numerous previous convictions dating back to 1973 for dishonesty and offences involving some degree of violence but no sexual matters.
His barrister Ian Brook said it was a “heinous” offence and Borrowdale understood the gravity of the crime and had expressed a degree of remorse.
Judge Jeremy Baker said the old lady was susceptible to strokes and had suffered a heart attack just three months previously and the stress of the assault had put her at further risk.
The judge told him: “Those who are subject to such an ordeal are unlikely to forget it and the psychological damage will be profound and long-lasting.”
It emerged after the hearing that Borrowdale had been in contact with the local mental health team after neighbours reported his odd behaviour but it was not deemed necessary to offer him treatment.