Edward Stewart, 53, set up a fake online profile in 2016, masquerading as a former member of the elite special-forces unit "to impress women", York Crown Court heard.
His profile claimed he had once been on SAS missions in Syria and Afghanistan and provided personal protection for Prince William and the Duchess of Cambridge.
He said he had stayed with Prince William and Kate to protect Prince George from a terrorist attack soon after he was born and also protected Brad Pitt and Princess Diana.
After his lies were exposed, he was welcomed back into the bosom of his family and moved in with his elderly father David Brunton, who trusted him to manage his finances.
But instead of looking after his ailing father, a World War Two veteran, Stewart cruelly and systematically rifled through his account after being handed his bank card.
The elderly widower was now a “broken man” and in poor health, the court heard.
Prosecutor Matthew Collins said Stewart stole tens of thousands from his trusting father, which came to light after his family carried out their own internal investigation.
There had been numerous withdrawals from Mr Brunton’s bank account, allegedly over several years, and Stewart was arrested after the police were called in.
He was charged with one count of fraud and three counts of theft but denied all allegations apart from one count of stealing £1,666 from his father during a four-week spree between June and July 2019.
He was due to face trial on the other allegations, but the family made a last-minute decision not to pursue these charges and they were allowed to lie on court file.
Stewart, of Harrogate, appeared for sentence on Wednesday (Jul 15) on the single count of theft he had admitted.
However, Mr Collins said this did not mean the family accepted he was innocent of the other alleged thefts. The remaining alleged stolen amounts would be pursued through the civil courts.
He said Mr Brunton, who served in the Grenadier Guards during the war, had recently been ill in hospital and his condition had considerably worsened since his son’s wicked betrayal.
Stewart had used his father’s bank card to make payments and withdrawals from cash machines, he added.
His sister Francesca Brunton launched her own investigation and Stewart admitted to his family that he had stolen the £1,666 in the summer of 2019.
Mr Collins told the court: “Repayment was arranged by direct debit at £50 per month." However, full repayment had still not been made and had now stopped.
The rest of the alleged stolen cash – said to be “tens of thousands of pounds” – had also allegedly been withdrawn from cash points.
Mr Collins said Stewart had been trusted to do errands such as shopping for his father, but he “abused that trust for his own personal gain”.
He had been welcomed back into the family after his lies exposed by internet vigilantes 'The Walter Mitty Hunters Club', which exposes people with fake military pretensions.
Stewart, a former hotel worker, hit national headlines in 2016 after he was named and shamed by the Facebook group. His boastful fake posts included one in which he claimed to have suffered a wound from a knife attack while protecting Brad Pitt.
He also claimed he had stayed with Prince William and Kate to protect their son Prince George from a terrorist attack soon after he was born, and said he knew Bear Grylls and talked about a burn on his chest from a ‘flash-bang’ injury during his 30 years of ‘military service’.
He said he had been on missions to Syria and claimed he had been made to kill a young Iraqi goat herder who had pointed an AK47 at him.
His father, a veteran soldier, had been "shocked and shamed" by his son’s mock military profile and "the blackening of his name in the press", the court heard.
Despite this, he forgave his son and put his trust in him once again after Stewart made a grovelling apology.
The subsequent betrayal, through the cash withdrawals, had an "extreme" effect on the decorated war hero.
Francesca Brunton, the victim’s daughter, said her father had suffered "mentally and physically" since Stewart’s "treachery".
Her ailing father had received daily calls from his bank and bailiffs had become involved after Stewart allegedly "falsified" a standing order on his account.
This had had a "devastating" effect on her father’s "already-fragile health" which had "steeply declined" and he was now a "broken man".
Abbi Whelan, for Stewart, said he had made attempts to repay the money and had lost his old job following his arrest.
He had found new work as a delivery driver but would lose that job and his home if he were jailed.
She added: “He is aware that he will never have any contact with his father again."
Judge Simon Hickey labelled Stewart a "complete Walter Mitty character" who had taken his father’s money for his "own selfish ends".
He told Stewart: “Your elderly and now-frail father is, in contrast to you, a man of impeccable character.
“He’s one of the few remaining veterans from the Second World War who, as such, should have been cherished by you and not defrauded in the way you had.
“You are a complete Walter Mitty character who posed as a SAS forces soldier, something your father would find abhorrent.
"It was against that background that you came to live with your family who remain devastated to this day.”
Stewart was jailed for eight months.