Ian Downs used modest-living Mary Garvey's money to fund his and his hairdresser wife's lifestyle, which saw them "living hopelessly beyond their means".
The 67-year-old admitted taking £90,565 from the widow's bank account before her death, between March 2005 and May 2013, and pocketing £19,697 paid into her bank from insurance policies, between May 2013 and October 2013, after she passed away.
It was only after the death of Mary, 94, who halted an explosion on an allied train during her war service, that the "startling picture" of deceit was revealed.
Father-of-one Downs, of St Bedes, East Boldon, pleaded guilty to two offences of theft at a previous hearing.
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Prosecutor Yvonne Taylor told Newcastle Crown Court: "Investigations showed that this defendant, at one time, worked for a building society and was believed to have financial expertise.
"However, he had not worked for many years and had not claimed benefits.
"His wife had worked as a part-time hairdresser.
"It is the prosecution case they were living hopelessly beyond their means, despite receiving a total of £38,000 in deposits into a joint account from the defendant's mother between the period of March 15, 2006, and July 2, 2008, after she died.
"Investigations also showed this defendant relied heavily on loans and credit cards.
"In essence, the complainant's money was eaten up by living expenses and debts."
Judge Sarah Mallett jailed Downs for two-and-half years.
The judge told him: "I have to bear in mind the importance of protecting vulnerable, elderly people from exploitation by family members who have easy access to their money."
The judge said it was an aggravating feature that the offending went on for over eight years and continued after the victim's death.
Judge Mallett said Downs had been living "beyond your means" but said there was no evidence of a "lavish lifestyle".
Prosecutors have started proceedings under the Proceeds of Crime Act to get back any remaining assets Downs has left.
Mrs Garvey had served in the British Army Catering Corps during the Second World War, where she met her soldier husband Anthony.
During her time in the military she travelled across Europe and was even credited with preventing the explosion of a bomb on an allied train.
Mary was sadly left widowed when Anthony died aged just 59 in 1977 and later became reliant on her family for support as she suffered from dementia.
Downs took control of her financial matters but secretly made a number of small payments into his own accounts.
He gave Mary, who lived in sheltered accommodation in Benton, Newcastle, money to spend each week and bought her groceries and clothing.
When the family asked for her to be moved into a care home set up especially for war veterans, they were told by Downs that it was not a good idea.
The deceit came to light months after Mary’s death when her daughter, and Downs' sister-in-law, Gwenda, and granddaughter Angela asked about any inheritance.
Downs told them he would let them know but months went by without any word and eventually Angela confronted him and demanded to see copies of the accounts.
When he failed to present the documents she contacted detectives at Northumbria Police who then uncovered the huge fraud.
Angela said her war-hero gran "deserved a life of luxury" but instead had been left without a penny to her name: "This case was never about the money.
"It was about the fact a woman who dedicated her life to her country was left to die in poverty.
"My grandmother deserved a life of luxury and should have had anything she wanted but instead a person she trusted ended up stealing all her money.
"She was vulnerable, lonely and suffering from dementia but my uncle took advantage of that for his own financial gain.
"He has torn our family apart and I will never be able to forgive him for how he treated my grandmother. I am glad that he will now be held accountable for his actions."
Fiona Lamb, defending, said Downs, who has health problems, had "always accepted" he stole cash but it was difficult to calculate exactly how much.
Miss Lamb said Downs has paid £32,000 back to the family and had not lived a life of luxury with the stolen cash.
She added: "This money was to put food in their mouths and keep a roof over their head."
Miss Lamb said Downs, who struggled financially after being made redundant in 2001, drives a 1996 Mercedes valued at just £50.
She said Downs' offending has "torn up an otherwise happy family" and that he is remorseful and ashamed.
Following the conviction, Detective Constable Frank Cox described the case as an "appalling breach of trust" and said Downs deserved to be put before the
Det Con Cox, who works in Northumbria Police’s Central CID, said: "Mary Garvey was a frail and vulnerable woman whose life was very much in the hands of Ian Downs.
"She had spent years dedicating her life to her country, she was a popular figure in Newcastle city centre as a pub landlady and her family were the most important thing to her.
"The defendant has taken advantage of his position of trust for the best part of a decade during which time he lived off the money he stole from Mary.
"In the weeks before he was due to appear in court he has attempted to pay some of it back but ultimately he has now been held responsible for his crimes.
"This case comes as a reminder of how low some people will sink if it means they can benefit themselves so if you have suspicions then we would urge you to contact police."
Speaking about her grandmother, Angela said Mary was a war hero who always put her family first and adored her husband Anthony, who had served in the Durham Light Infantry during the Second World War.
Angela said: "My grandma was a lovely lady who had suffered from dementia in her later years but served honourably in the Second World War.
"Throughout her life she was a strong and purposeful woman who had been through much but had nevertheless married her wartime sweetheart.
"She went on to be a mother of two girls and a grandmother to two more. I loved her, and still do love her, very much.
"Her values were to work hard, save for the future and put her family first.
"She and my grandad worked hard and earned every penny they ever had.
"They didn't have it easy and never squandered what they earned. My grandad knew that through work came self-sufficiency and self-esteem.
"I'm blessed I had the opportunity to tell her she was going to become a great-grandmother for the first time. I will never forget that day and that conversation.
"Sadly she passed away shortly afterwards before she was able to hold her great-grandson, I'm sure she would have loved him."