Martin Jacks, 41, of Greystoke Drive, Bilborough, was caught by police as he tried to flee the pensioner's home.
Lincoln Crown Court heard Jacks was serving a two year suspended jail sentence for similar offences in 2012 and 2014 when he carried out the fraud on December 18, 2017.
Jacks pleaded guilty to a single charge of fraud by false representation as the company he claimed to work for did not exist.
When police arrived at his elderly victim's home for other reasons, they discovered he had attempted and failed to write three cheques.
The court heard Jacks' victim was diagnosed with dementia a year after the offence.
“I am satisfied he had memory issues,” Judge Catarina Sjolin Knight told Jacks. “He was clearly very vulnerable.”
Daniel Brayley, prosecuting for Trading Standards, told the court: "The defendant targeted a vulnerable victim. That firm didn't exist, so the work wouldn't be happening."
The court heard Jacks received a 24 month suspended jail sentence in June 2016 for three fraud offences dating between 2012 and 2014.
“There was a history of offending here. It wasn't an isolated incident.”
Simon Eckersley, mitigating, told the court Jacks had stayed out of trouble since 2017 and was working in an office based marketing role for an established company.
Mr Eckersley argued the matter was almost five years old. “It has afforded Mr Jacks the opportunity to prove he has removed himself from this sort of behaviour.”
Mr Eckersley said it was also fortunate in this case that victim had been unable to sign any money over.
Judge Sjolin Knight told Jacks she was unable to pass a suspended sentence because of his previous offences.
"This is a return of offending against a vulnerable person," Judge Sjolin Knight said.
After the hearing, Martyn Parker, assistant director for public protection at Lincolnshire County Council, added: “This case also highlights the close working relationship between Lincolnshire Police and Lincolnshire Trading Standards; both organisations work tirelessly to protect vulnerable residents like Mr A and stop the criminals who choose to take advantage of them.”
If you are concerned that a business is acting in a way that they should not, you can report this to Trading Standards through the Citizens Advice consumer service. Visit www.citizensadvice.org.uk/consumer or call 0808 223 1133.