Ricky Gray reportedly stole £250,000 of jewellery from Patsy Bloom when he burgled her £3.5 million house overlooking Regent's Park in London.
Miss Bloom was watching television in bed when Gray, 30, burst in, shouting: "Give me everything you've got."
The 72-year-old was left "extremely shaken" after the burglar marched her to her safe and made off with her valuables, including a £68,000 pair of 18-carat white gold earrings and a £19,000 diamond pendant.
The burglary was among a string of crimes carried out by Gray to fund his cocaine addiction, the Daily Mail reported.
It came two months after he stole an £11,000 platinum diamond solitaire ring from a jewellery shop near the London Stock Exchange.
Miss Bloom made a £16 million fortune from the sale of Petplan, the animal insurance company she had set up more than three decades earlier.
Robert Hutchinson, prosecuting, told Blackfriars Crown Court in London: "After showering, (Miss Bloom) got into bed in order to watch the news and use her iPad.
"As she did so, her bedroom door opened slowly, and the defendant came into the room. He had a knife, which he was pointing at her.
"He was using both hands to hold it, and used something to cover the handle."
Judge Henry Blacksell QC described Gray as "uncontrolled" and ready to resort to "desperate measures".
"This was a lone woman, it was her home, and she was entitled to be in her home and feel she was safe," he told Gray.
"She was abused by you and terrified into giving you her property. It was audacious, and that is why you are a risk to the general public."
He sentenced him to eight years in prison but Gray will automatically be released after serving only four.
As he was led away, he called out "Cheers, thank you."
The burglar has committed a total of 61 offences including muggings, burglaries and attacks.
Avi Chaudhury, defending, said he stole to buy cannabis, ecstasy and cocaine.