Adam Mackie, 19, held a blade to his boss's throat before the scarf covering his face slipped in the "terrifying" hold-up at the Iceland shop.
Mackie, from Caerphilly - who had worked there for five months - later admitted robbery.
Judge Thomas Crowther QC jailed him at Cardiff Crown court on Wednesday.
The court heard how Mackie targeted the Cardiff Road store in the early hours in March knowing there would be just a few colleagues stacking shelves.
Prosecutor Tony Trigg said Mackie repeatedly told the duty manager he would "have his throat slit" unless Â£1,000 was stuffed in a bag.
"It was a clearly terrifying incident involving the use of knives," he added.
But Mackie was recognised when part of his face was revealed during his escape and he was later arrested.
Judge Crowther said Mackie "hated" the duty manager and "seemed to relish the opportunity to 'give him a kicking'."
A 15-year-old boy who was Mackie's accomplice was handed a two-year detention and training order.
A Caerphilly shop worker who donned a mask and carried out a knifepoint raid at his own store has been jailed for eight years.