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.
A man who defrauded a 98-year-old woman he befriended of £170,000 has been jailed for five years and one month.