Sally-Ann Corby was part of a sophisticated gang of fraudsters which, armed with inside information, stole pounds 490,000 from the account of Joan Lee.
Corby, 55, wore a grey wig for her visits to Barclays bank in New Malden, south-west London. On her final visit she arrived in a chauffeur-driven limousine and left with pounds 300,000 of Mrs Lee's cash and pounds 150,000 of her jewellery.
The court heard that Corby, who had already extracted pounds 40,000 before the final visit and was receiving income support, played her role with "considerable panache".
Passing sentence the Recorder of London, Sir Lawrence Verney, told her: "Having made yourself up to look like the account holder and successfully practised her signature, you presented yourself to the bank and deceived the manager very easily. You exhibited great composure, whatever you felt internally."
Corby was involved in another swindle after recruiting Gwen Cox, 39, a boatyard manager. They took pounds 30,000 from the account of a wealthy New York recluse, Lady Monique Caro, before being caught.
Cox, who received just pounds 1,000 from the gang, was sentenced to 18 months imprisonment, suspended for 18 months, after admitting two charges of theft and one of attempted theft.
Corby, who had been recruited to the gang by a family friend, wanted to use the pounds 50,000 she received for her role to renovate her derelict cottage in Northam, north Devon.
The scheme's alleged mastermind is still on the run.