Surinder Singh, 42, was found guilty of robbery and possession of a bladed article at Wood Green Court in London.
Scotland Yard described the man as a "career criminal" who resorted to robbery to fuel his drug addiction.
"Singh is a career criminal who uses extreme violence for relatively little reward to fuel his drug addiction. The terrified victim genuinely believed that he would be killed by the knife wielding attacker if he did not hand over cash.
Judge G Perrins described Singh's attack as a "despicable crime" as the victim trusted Singh and thought he was his friend.
More From This Section
As Singh was clearly motivated by his heroin and alcohol addictions, the judge said he felt there was no alternative but to hand down a substantial sentence based on Singh's offending history and his propensity to commit violence.
Petrified and believing Singh would kill him, the victim handed over 80 pounds (USD 99.99) before fleeing the scene.
During Singh's police interview, he denied the allegations put to him, claiming that they were fabricated by the victim.