The judge at Carlisle Crown Court said "devious and selfish" Donald Scott, 62, from Wigton, abused his position of trust to take money from his customers to pay off his own debts.
He admitted 13 charges of fraud.
Scott forged signatures to sell off his clients' land entitlements and kept the profit for himself.
Judge Peter Davies told Scott a custodial sentence was inevitable because of the vulnerability of his elderly victims.
He said: "It's not a just a case of money, it goes beyond that. It was their businesses, their lives were affected, their health was affected.
"They entrusted you with large sums of money and you betrayed that trust."
The court heard Scott's victims trusted him with their retirement funds and paid him to help them claim EU subsidies.
But, as his business fell into debt, he started selling off their land entitlements which they had used to claim the money.
He also told clients he could get them good returns on investments but he took the money to pay off debts.
One victim, a retired farmer, lost all his savings of £200,000.
Scott committed the crimes between 2010 and 2015.
Speaking after Scott was jailed, Det Con Duncan Watson, from Cumbria Police, said: "It's a despicable crime."
An estate agent has been jailed for five years for taking more than £450,000 from elderly clients in Cumbria.