Anne Peterson, 59, was found guilty of four counts of fraud while working at Lloyds Bank in Barry.
She told Cardiff Crown Court that she was on holiday when the transactions took place and that somebody else had used her bank log in details.
Judge Stephen Hopkins adjourned sentencing until 23 June.
He told Peterson that she had been convicted on "compelling and overwhelming evidence" and that she "ought to be thoroughly ashamed" of herself.
Peterson withdrew £12,000 from the account belonging to a 92-year-old woman, £4,000 of which was just a fortnight after her death.
Her second victim was an 83-year-old man, who had £5,080 taken from his account.
The court heard Peterson diverted their statements so they would not be able to find out what was going on.
A Vale of Glamorgan bank worker faces jail for withdrawing more than £17,000 from the accounts of two customers, one of which had died.