A regular customer at a Post Office brandished a rusty screwdriver and told terrified staff 'I've already killed someone today'.
Steven Power, 56, who went in the convenience store on almost a daily basis, demanded that they hand over 'all the money'.
He threatened to stab them before helping himself to cash from the till.
The shop, which was being run by three women that day, lost more than £6,000.
"I can't believe someone I recognised as a customer has done this," one worker said.
While another added: "I don't trust anyone now, even if they were a regular and really needed help."
Power had come into the shop and Post Office on Beswick Street in Ancoats, at about 5.15pm on October 13 last year.
He told staff he had 'something to tell them', asking them to come out from behind a closed off staff area.
He then pulled out the screwdriver and said 'I've already killed someone today', before demanding money.
One worker tried to push him away, but Power shoved her which caused her to stumble.
They allowed him access to the cash drawer, and a check later revealed they'd lost £6,650.
Power left the shop and got into a car, prosecutor Saul Brody told Manchester Crown Court.
About 45 minutes later, police were flagged down near a building site.
They found Power lying on the ground and 'unresponsive', with the screwdriver but no cash.
He later told police he had been suffering from health problems and 'didn't feel well', admitting he'd been drinking and couldn't remember what happened.
One shop worker said they were 'terrified', while another felt 'stupid' after the robbery.
Power has a previous conviction from more than two decades ago for the attempted robbery of a Post Office, the court heard.
Now he has been jailed for three-and-a-half years, after admitting robbery and possessing an offensive weapon.
Defending, Julian Goode said the robbery was 'poorly conceived', and Power had not used a disguise.
He has 'no recollection' of what happened, the court heard.
Mr Goode said Power, who has had drink and drug problems, had 'engaged incredibly well' with support services and has expressed 'deep remorse' for his actions.
"It must have been, and clearly was for those involved, a terrifying ordeal," Judge Timothy Smith said.
"They (the shop workers) knew exactly who you were, they were feeling they could trust you as a customer and a regular."
