Options I know of are driving there, or flying to someplace where one might rent a car, and then driving. The closest place I know I can fly to is San Francisco, which is about 4 hours' drive away. I see there are little airports near or in Mendocino... is it possible to schedule a flight there? At reasonable expense? Or is Oakland and/or driving self the best way?
Rome2Rio suggests you fly to Santa Rosa, which is well north of the Bay Area, and almost halfway between San Francisco and Mendocino.
Alaska Airlines flies from SEA-STS, though for most flights you'll have a layover in PDX (I'd try to avoid this if possible). Direct flights are available, but apparently not every day of the week. And this is the only commercial airline serving that airport.