Julie Sayles, 59, of Bridlington, East Yorkshire, took advantage of "frugal" Edith Negus after discovering the centenarian had saved a "considerable fortune" during her lifetime.
Sayles, who worked for a charity providing care for the elderly, bought two houses with the money and wrote herself into a new will in which the majority of Ms Negus's remaining assets were left to her.
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.
Sentencing her on Monday afternoon, Recorder Anthony Kelbrick said Sayles had coveted the wealth Ms Negus had amassed through hard work and saving.
He said: "(Edith Negus) lived through the reigns of three kings and a queen, she was born before two world wars and survived twice being bombed in London.
"When she was born, horse-drawn vehicles were still being used on our streets, televisions had not been invented, computers were a thing of the future, there was no welfare state.
"It was a different world and it produced different people.
"She had a strong work ethic and a strong save ethic. She was frugal, she wasn't lavish. She and her husband must have saved much of what they earned."
He continued: "You became aware this was no poor, old lady with no assets but a lady who had saved a considerable fortune.
"You coveted that wealth and set about acquiring it for yourself."
Recorder Kelbrick added: "For merciless fraudsters like you there can be only one sentence: prison."
Ms Negus began receiving care from Friends of the Elderly Bridlington, a charity run by Sayles and with no connection to the national charity of the same name, after her physical and mental health began to deteriorate and her family were unable to provide care due to their own health issues.
Recorder Kelbrick said: "They believed you, Julie Sayles, had taken over the mantle of caring for Edith Negus.
"They trusted you and Edith Negus trusted you."
In 2014, Sayles set up a joint account with Ms Negus, into which transfers were made from the elderly woman's savings accounts.
Between February and July 2014, Sayles made a number of withdrawals totalling Â£287,688, which she used to buy properties in Trowbridge, Wiltshire, and Scarborough, North Yorkshire.
Recorder Kelbrick said: "You took advantage of her frailty time and time again."
The authorities became involved after a neighbour overheard Sayles talking to Ms Negus about making a new will leaving her estate to her.
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.
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's assets were left to the carer and which referred to Â£250,000 already gifted to her as a thank-you.
Sayles, who was seen laughing and joking at the funeral she organised for Ms Negus, was later arrested.
She denied any wrongdoing throughout her police interviews - claiming Ms Negus had offered her the money and that the wills were created on her instructions.
Speaking outside court after the sentencing, Ann Ruthven, Ms Negus's great niece, said they had been unable to grieve properly for Ms Negus and said the situation had caused them "stress, anxiety and heartache".
They described Ms Negus as a "beautiful, kind, warm and loving woman with many friends and family who loved her dearly" and asked for her to be remembered as "caring, smiling, full of life and always calling you darling".