Jake Griffin, 22, from the St Leonards area but of no fixed address, was sentenced o at Lewes Crown Court after handing himself in to officers.
He pleaded guilty to two counts of robbery, one count of having an offensive weapon, one count of driving a vehicle without consent and two counts of possession with intent to supply class A drugs.
On one occassion he went into a shop armed with a 10 inch kitchen knife with an accomplice. Both men had their faces covered and demanded that the female member of staff open the cash till.
The men escaped with around £100 and cigarettes. The woman was left very shaken by the whole incident.
Just a few days later, the same two men, again with their faces covered, entered another convenience store in St Leonards-on-Sea and confronted the manager as he came out of the staff toilet.
The victim was threatened with a screw driver and told he would have his throat cut if he didn't open the till.
When he explained that the till was on a time lock the suspects demanded he open the cigarette display cabinet.
The manager tried to shut himself in the toilet but the door was forced open and he was dragged to the cigarette counter.
The suspects were unable to open the till and left empty handed on a stolen motorcycle. The victim was so affected by his ordeal that he has since been relocated to another store.
Detective Constable Mark Waughman, who led the investigation said: "I am pleased with the sentence that Griffin received - it reflects the seriousness of his offences and how frightening it was for his victims, who were not to know whether he would carry out his threats to harm them.
"I hope his long sentence will be of some consolation to them and act as a strong deterrent to anyone else who is thinking of committing similar crimes in the community."
Detective Inspector Mark O'Brien, from East Sussex CID, said: "It sends out a very clear message that violent knife crime will not be tolerated by society, and the courts will pass appropriate sentences that clearly reflects public concern.
"The crimes reported are thankfully isolated, and police are committed to reducing all violent crime and in particular knife crime."