The fire was reported at about 2:30 p.m., according to Contra Costa County Fire Protection District Inspector Steve Aubert.
Plumes of smoke from the blaze were visible in Antioch, but Contra Costa County crews were called away from the scene because Kimball Island is not in the county's jurisdiction, Aubert said.
Coast Guard crews responded to assist in evacuating at least four people off of the island, Lt. j.g. Corinne Gaines said.
Three homes, several outbuildings and 40-50 acres were burned before Solano firefighters cleared the scene just before 9:30 p.m., a Solano fire dispatcher said.
The fire started as a 10-acre brush fire but grew rapidly because of strong wind, according to the Coast Guard.