Rockslides triggered by the shaking left some roads on the island blocked while homes and stores suffered cracks and broken windows.
The Athens Observatory said the temblor struck at 3:55 p.m. local time about 180 miles west of the capital at a depth of 15 miles beneath the Kefalonia town of Lixoúrion.
The shaking was felt on several Ionian islands and as far away as Athens.
Many residents near the epicenter slept outdoors Sunday night, and workers at the local airport refused to enter the terminal out of fear of an even stronger quake.
Kefalonia has been hit by powerful quakes during its recorded history.
A major temblor in August 1953 destroyed virtually every house on the island.