Julie Sayles, 59, bought two houses with the money she transferred from a joint bank account she set up with 'vulnerable' Edith Negus.
Sentencing her, Recorder Anthony Kelbrick said: "In interviews with the police and here to your jury, you told lie after lie after lie.
"Seeking always to answer the questions put to you, you suggested all that happened was instigated by and represented the wishes of the lady you called your friend and for whom you said you wept.
"For merciless fraudsters like you, there can only be one sentence – prison."
'Vulnerable' Edith Negus' health began to deteriorate after her 100th birthday
A jury at Hull Crown Court took around two hours to find the defendant guilty of fraud by abuse of position, buying properties with the proceeds of crime, making a fraudulent will and presenting it to a solicitor.
As the unanimous verdicts were read out, Sayles looked towards her family, who left the courtroom in tears.
During the six-day trial, the court heard that Ms Negus' physical and mental health began to deteriorate after her 100th birthday.
She began receiving care from Friends of the Elderly Bridlington, a charity run by Sayles, who was described as 'domineering'.
Robert Stevenson, prosecuting, told the jury that Ms Negus had become 'vulnerable' and that Sayles 'took advantage of her'.
Sayles set up a joint account with Ms Negus and helped herself to cash
The trial heard how Sayles set up a joint account with Ms Negus, who had saved a large amount of money during her lifetime and was described as 'thrifty'.
Transfers were made into that account from Ms Negus's savings accounts and, between February and July 2014, Sayles made a number of withdrawals totalling £287,688.
The court heard that she used the money to buy two properties, in Trowbridge, Wiltshire, and Scarborough, North Yorkshire.
Ms Negus, who had no children but treated one niece as her own daughter, had a will in which she left her belongings to her family.
In the days after Edith's death, Sayles presented two new wills to a solicitor
But, in the days after her death, in October 2014, Sayles presented two new wills to a solicitor, in which the majority of Ms Negus' assets were left to the carer and which referred to £250,000 already gifted to her as a thank-you.
The authorities became involved after a neighbour overheard Sayles from Bridlington, East Yorks., talking to Ms Negus about making a new will leaving her estate to her.
Speaking about Edith, the recorder told Hull Crown Court: "She lived through the reign of three Kings and Queens and was born before two world wars and survived being bombed in London.
"When she was born horse drawn vehicles were still being used on our streets, TVs had not been invented and computers were a thing of the future.
Sayles was found guilty of fraud and jailed for nine years
"There was no welfare state. It was a different world and it produced different people. She (Edith) had a strong work ethic and a strong save ethic.
"She never realised, perhaps, she could and should have enjoyed some of the pleasures her hard earned and hard saved cash could have brought her."
In a victim impact statement read to the court, Ms Negus' family said they had been unable to grieve properly and said the situation had caused them 'stress, anxiety and heartache'.
They described Ms Negus as a 'beautiful, kind, warm and loving woman who had many friends and family who loved her dearly'.