41-year-old Jamie Neil initially thought the bag would mask his identity, but his face was clearly captured on CCTV.
Neil and his accomplice, 20-year-old Gareth Tilley, had consumed alcohol and stolen medication before they decided to rob the petrol station in St Austell, in Cornwall.
Tilley wrapped a scarf around his face, and pretended his mobile phone was a gun, pointing it at the cashier and demanding money - but was found out when the keypad lit up and the lone female assistant pressed an alarm.
Neil then wrestled with the worker and headbutted her before fleeing with a number of bottles of spirits.
Police studied CCTV footage following the raid and Neil was recognised in the street some days later by an off-duty officer, who called for backup.
Detective Constable Steve White said the disguise was the most “ridiculous” he had ever come across.
He said: “Before arriving at the location Tilley disguised his face using a scarf and Neil, being less resourceful, put a plastic bag on his head.
“I investigated the case with CCTV and forensic evidence used to identify the assailants.”
The 41-year-old Neil was convicted of robbery last Friday and jailed for two years. Tilley had earlier pled guilty and was also jailed for two years.