The store, Express Lane Marathon at 17200 Robbins Road, was temporarily closed Saturday night because power was shut off due to the electrical fire.
The Grand Haven Department of Public Safety was called to the store shortly after 5:30 p.m. when a clerk and a customer smelled smoke, said Jeff Kriger, the department's fire investigator.
"It burned through a 2-by-4 (stud), so it's been going for a while," Kriger said. "We cut the power to the building. They'll have to have an electrician come out here."
GHDPS officers doused the fire with water. Kriger said no one was hurt, and there was no damage to the building other than the sign and facade.
GHDPS was called to the fire as the city and township mutually respond to that area. The Pizza Hut restaurant next door to the west and the D&W plaza across the street to the east are both in the city, while the gas station is in the township.