Samantha Davies entered the man's home in the early hours of the morning using a key she had stolen earlier.
She walked into the 65-year-old man's bedroom where he was asleep and called out his name to wake him.
Leanne Summers, prosecuting at Lincoln Crown Court, said Davies asked the victim to sit up and then she removed a gold necklace from around his neck.
Davies also took a gold bracelet off his wrist and unsuccessfully tried to remove the watch he was wearing.
She then made off with the necklace and bracelet which were together worth Â£2,000.
Miss Summers said Davies had entered the house on Lea Road, Gainsborough, at 4.30am using a front door key she stole during a burglary she carried out on the same house earlier the same evening.
In the first raid she took a bank card, a cheque book, a mobile phone and cigarettes. She later used the bank card to buy almost Â£90 worth of goods from the Jet petrol station in Gainsborough.
The victim later told a neighbour what had happened to him and police were called.
Miss Summers said police had previously installed security cameras inside the man's home because of earlier issues and Davies was identified from the CCTV film which showed her inside the property.
Miss Summers said: "The complainant is 65-years-old. He has a number of health issues making him vulnerable.
"He was involved in a fall and suffered a severe blow to the brain. It left him with memory problems. He has also been provided with a mobility scooter to help him get around."
When Davies was arrested she admitted she had visited a number of times but claimed she did so to check on his welfare.
Miss Summers said: "She denied taking advantage of him. She denied the burglaries and said she was elsewhere at the time."
Davies, 31, formerly of Gainsborough but currently of no fixed address, later admitted two charges of burglary as a result of the incidents on June 17 this year. She also admitted three charges of fraudulently using the man's bank card.
The court was told Davies had eight previous convictions for a total of 20 offences.
Judge Simon Hirst, passing sentence, told her: "This was a disgraceful series of offending. It was mean and nasty.
"You took advantage of a vulnerable man having wormed your way into his friendship. He was deliberately targeted."
David Eager, in mitigation, said: "She has a depressing history of convictions. They are the sort of convictions you see from those who are addicted to heroin.
"She can only explain her behaviour by the need to obtain heroin."
He said Davies intends to stay off drugs and move from the Gainsborough area when she is released from her sentence.