Julie Sayles from Bridlington used the money she took from Edith Negus to buy two houses. The 59 year old also wrote herself into a new will in which the majority of Ms Negus' remaining assets were left to her.
Sentencing her on Monday afternoon, Recorder Anthony Kelbrick at Hull Crown Court said: "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. For merciless fraudsters like you there can be only one sentence: prison."
Sayles was previously found guilty at Hull Crown Court of fraud by abuse of position, buying properties with the proceeds of crime, making a fraudulent will and presenting it to a solicitor.
She made withdrawals totaling £287,688 between February and July 2014 which she used to purchase properties in Wiltshire and North Yorkshire.
Sayles founded Friends of the Elderly Bridlington, a charity offering care for the elderly. The organisation has no connection to a national charity of the same name.
Ms Negus - who died in October 2014 - received care from Sayles' charity after her health deteriorated and her family were no longer about to care for her.
Detective Superintendent Matt Baldwin, of Humberside Police said: "She [Ms Negus] was betrayed by Julie Sayles, who saw an opportunity to steal the life savings of the victim for her own benefit. In doing so, she deprived the victim from leaving the money to her family.
"The jury's decision has now given the family some justice for their loved one and will hopefully provide them with an opportunity to grieve and remember Edith for the right reasons."