Jacob Simpkin, 24, grabbed the woman from behind, held a 30cm knife to her throat and demanded to be taken to the safe at the shop in Peterborough.
She led him there, and as her colleague opened the safe and placed around £7,000 in a black sports bag, she caught a glimpse of Simpkin's face.
The woman confronted Simpkin, who begged her not to tell police then fled with the cash on December 20 last year.
He was arrested the following day and later admitted robbery at Cambridge Crown Court, Cambridgeshire Police said.
Simpkin, of Appleyard, Peterborough, was jailed for four years on Monday.
Detective Constable Stuart Cooper said: "This was a terrifying experience for the people in the shop; thankfully nobody was hurt.
"I'd like to thank the witness who recognised Simpkin for having the courage to come forward and report what she knew."