Gordon Beurskens claimed pensioners Jimmy and Nettie McQue had given him "unlimited" control of their bank accounts so they could keep their money "below the radar".
He said the £77,580 he took from the octogenarians was an ongoing "loan" which he intended to repay with interest.
But a jury at Livingston Sheriff Court rejected his excuses and returned a majority verdict finding him guilty of embezzlement.
Moments after his conviction the former West Lothian Independent councillor was hit with a Proceeds of Crime demand from the Crown.
Former ironmonger Jimmy McQue - who suffered from dementia - died several years ago.
The jury heard evidence from a police statement given by his wife Nettie, now 86, in 2008. In it, she said that she trusted Beurskens so much she would "sign anything" he put in front of her.
She had told detectives: "I have been gullible. I think Gordon has taken advantage of us. I would have signed anything if Gordon had asked as I had trusted him. Having looked at what he has done, Gordon has abused our trust when we thought he was being kind."
The jury heard Beurskens used the couple's cash to run up a huge bill at the Ritz Hotel in Piccadilly, central London.
Beurskens, 51, was earlier found not guilty of money laundering after the prosecution withdrew the charge.
Sheriff Kevin Drummond called for background reports and told Beurskens he would be sentenced later. Bail was continued.