Paul Storm Blueitt, 36, of Cambridge Crescent, East Dene, Rotherham, attacked 64-year-old Judith Ducker in Wellgate News in Rotherham town centre on September 1 last year.
Mrs Ducker was left with a fractured skull, a fractured eye socket, multiple head lacerations and bruising to the brain in the attack before Blueitt stole £40 and fled.
Blueitt, who was found guilty after a trial, was sentenced to life behind bars and was ordered to serve a minimum of 27 years before he can be considered for parole.
Police incident Woodhouse Sheffield: Two men and a woman injured after large fight breaks out on Market Square
Sheffield fire: Crews tackle blaze in Wharncliffe Woods near Oughtibridge as heatwave continues
Sheffield fire: Video shows firefighters battling blaze at Wharncliffe Woods near Oughtibridge
Sheffield Crown Court: Here are the biggest cases from the last week including paedophile sting
Sheffield father poured petrol over daughter and tried to set her on fire
Detective Constable Mick Walker: said: “Mrs Ducker was left with serious head injuries after she was assaulted by Blueitt during a robbery at Wellgate News in Rotherham on 1 September last year.
“At the time of the incident, Judith was living a normal life, despite living with breast cancer that had spread to other parts of her body. Oncologists from both Sheffield and Rotherham hospitals were managing her cancer and she had responded well to the treatment.
“Blueitt entered the shop shortly after 11am that morning and subjected Judith to an horrendous assault, during which she sustained serious head injuries.”
"Mrs Ducker was left with a fractured skull, a fractured eye socket, multiple head lacerations and bruising to the brain. Blueitt stole £40 from the till before leaving the shop on a bike.
DS Andy Shields added: “The consequence of this assault, was that Judith would never be well enough to receive further cancer treatment and after being taken to hospital, a CT head scan revealed that Judith’s breast cancer had spread to her brain.
"Such were her head injuries that further cancer treatment could not be given to her and she sadly died in hospital on October 20 last year. She died from the breast cancer that had spread to her brain.”
Blueitt was initially charged with attempted murder but after Mrs Ducker’s death, the charge was upgraded to murder, following consultation with the Crown Prosecution Service.
The prosecution case was that the head injuries caused by Blueitt prevented Mrs Ducker from receiving vital cancer treatment and resulted in her premature death.
“This has been an incredibly sad and difficult time for Judith’s family and friends, who have all shown immense courage, understanding and patience throughout the investigation and the trial, and I would like to thank them for that,” added DS Shields.
“I would also like to thank all of the medical professionals from The Rotherham, Sheffield and Leeds NHS Trusts without whom this prosecution would not have been possible. Despite their professional commitments, they found the time to meet with the investigating officers and to attend the trial to give expert and medical evidence.
“This was a complex investigation, and the conviction has come through the hard work of the investigative team, alongside input and support from key partner agencies.
"I hope that this offers some degree of closure to Judith’s family who have had to endure the agony of having their loved one taken away from them.”