Russell Joy, 45, of Shapwick, Dorset, set up direct debits, withdrew money and cashed bonds on behalf of Mary Brady, from Broadstone.
Bournemouth Crown Court heard Joy won the trust of the pensioner, who died earlier this month, when she hired him as her gardener in 2008.
He was entrusted to withdraw money for her daily living expenses.
In December 2014 he made the first of 107 withdrawals without her knowledge, totalling £32,100.
The following November Ms Brady cashed five bonds in the sum of £230,000 and wrote Joy a cheque for £100,000.
In March 2016, he received a further £20,000 cheque.
He also withdrew money to buy a Land Rover car and set up a direct debit to pay his vehicle insurance.
The offences came to light when the pensioner's relatives checked her finances while she was in hospital.
In a statement, Ms Brady's family said they were "desperately sad" about the "ugly end" to her life.
Det Insp Andy Dilworth said: "Russell Joy is nothing more than a callous fraudster. He befriended his victim over a period of many years and quickly earned her trust."
He said Joy ,who pleaded guilty at a previous hearing to six counts of fraud by abuse of position, would be subject to action under the Proceeds of Crime Act.

Sian Hollands, 25, suffered a pulmonary embolism after leaving Darent Valley Hospital, Dartford, in November 2015.
Dr Kamran Khan previously claimed he had not seen Ms Hollands' notes and was unaware of the seriousness of her pain.
However, during cross-examination he admitted having the notes but not reading them.
The General Medical Council is now investigating him.
Ms Hollands, a mother of three, went to Darent Valley Hospital complaining of breathing difficulties and chest and stomach pain. She had suffered an ectopic pregnancy three weeks earlier.
Gravesend Coroner's Court heard the former drug user was taking methadone to treat her addiction, but had stopped for a few days.
Ms Hollands was given an electrocardiogram (ECG) and methadone, and was given her discharge papers.
Her former partner Carl Alleyne said she was in pain and not breathing properly, but was told the hospital "needed the bed space".
He refused to take her home and she died a few hours later from a blockage in the artery carrying blood from the heart to the lungs.
Dr Khan put her symptoms down to drug withdrawal, the inquest heard.
He claimed not to have seen the medical notes labelling her as "at risk" and detailing her chest pain, which would have made him consider a pulmonary embolism.
But the medical notes were found with Dr Khan's own writing on the back, proving he had seen, but not properly read, the vital information on the paper.
Ms Hollands' family's barrister, Edward Ramsay, asked the doctor: "It means you could not have read what was staring you in the face?"
"Yes," he replied.
"Simply turning over the page and Sian may well be alive - do you accept that?" Mr Ramsey added. Dr Khan agreed.
Mr Ramsay said Ms Hollands was "labelled a heroin addict and nobody considered any other diagnosis".
Her mother, Nicole Smith, said that more than 500 days since her daughter's death, they were finally getting the truth.
"She was recovering, trying to put her life right, and they condemned her and stopped her from ever moving forward."
A spokesman for Darent Valley Hospital said: "We extend our sincere condolences to the family and friends of Sian Hollands. We await the outcome of the coroner's inquest."
The inquest has been adjourned until 10 April.
A man who defrauded a 98-year-old woman he befriended of £170,000 has been jailed for five years and one month.
[NEXT_CONCEPT]
A mother who died hours after being given her hospital discharge papers may have lived if a doctor had read her medical notes, an inquest heard.