The fire was reported shortly before noon at Gross Lighting at 201 West 4th Street.
Nearby buildings, including the Wilmington Campus of Delaware Technical and Community College, were evacuated as the fire went to two alarms.
The fire was placed under control at 2:06 p.m.
Fire officials say the building had been condemned and deemed uninhabitable by the Department of License and Inspection in 2012.
No civilian injuries were reported. Two firefighters sustained minor, non-life threatening injuries and were transported to Wilmington Hospital for treatment.
An investigation into the cause of the fire continues.