The attack happened yesterday (Tues) at the Texaco Service Station in Muff shortly before 5.30pm.
The thief made his getaway in a white Transit van which has a registration number of LHZ 1696.
He was wearing a hi-viz yellow workman’s jacket, dark overalls and white runners.
The van was driven in the direction of Muff before heading towards Shantallow in Derry and being abandoned on Carnhill.
The garage assistant was shaken after the incident but was physically unhurt.
The owner of the garage has offered a reward for information on the robbery which may lead to an arrest.
Both the Gardai and the PSNI are investigating the incident.
It is understood that Gardai are viewing good quality CCTv footage of the raid.