The robbery happened at the 7-Eleven store at 1647 North Broad Street in Hatfield Township around 9:30 p.m. on Saturday.
Investigators say, before the robbery, the suspect was inside the store in normal street clothes.
Police say he eventually left the store only to return a few minutes later wearing a ski mask and armed with a handgun.
However, he was caught on surveillance camera in both instances, police say, giving investigators a before-and-after shot of the suspect.
Once inside the store, and wearing the mask, police say the man ordered the clerk to hand over the cash in the register.
He then allegedly told the clerk to lie on the ground as he made his getaway.
A witness described the suspect to police as a man standing about six feet tall who got into a black Ford Taurus.
The car then fled east toward Route 309.
Anyone with information is asked to call the Hatfield Township Police Department at 215-855-0903.
