Police have released CCTV images following the robbery at the Texaco Garage in Regent Road at 1am on Monday December 10.
A man entered the garage, pulled out a knife and threatened a cashier, making demands for money.
The cashier then opened the till and handed over a quantity of cash.
PC Stu Kane from Lancaster Police said: “Despite the man working in the garage not being physically hurt, he has understandably been left shaken up by this ordeal.
“While the images don’t show the man’s face, the clothing is quite distinctive and so I would ask anyone who recognises it or has any information to get in touch.”
The man is described as around 6ft 2ins tall and was wearing a black ski mask, a red and black cap, a Crombie-style overcoat and purple plastic gloves.
He spoke with a heavy, gruff voice.
Anyone with information can email [email protected] or call 01524 596451.
If you fail to get an answer, call 101 quoting incident reference 0040 of December 10.
Alternatively, independent charity Crimestoppers can be contacted anonymously on 0800 555111 or online at crimestoppers-uk.org.