The Ukiah Daily Journal reports that the county's Board of Supervisors on Tuesday voted unanimously to make the declaration, citing an "imminent threat of disaster" from drought conditions.
The state Department of Water Resources said Mendocino is the first to request state drought assistance.
Officials hope the declaration will result in state funds and assistance in developing a drought plan.
Gov. Jerry Brown has convened a task force to help decide if a statewide drought emergency is warranted.
State water managers say they will be able to deliver just 5 percent of the water sought by municipal agencies and nearly a million acres of farmland.
Four consecutive years of drought, millions of dead trees and summer heat, are all factors as thousands of firefighters work to control wildfires in California.
Not much change is expected in drought conditions in California during the summer "dry season" but wildfire danger is increasing, with 66 million dead trees in the Sierra Nevada adding potential fuel.
The U.S. Forest Service says 66 million trees are dead in the Sierra Nevada after four consecutive years of drought in California and a bark beetle infestation.
California is in the fifth consecutive year of drought and water providers continue to urge voluntary conservation, as mandatory statewide rules have ended. Sacramento-area residents reduced their water use by 31 percent in May.
As thousands of federal and state firefighters work to contain wildfires in California and other western states, record-setting heat has prompted warnings from the National Weather Service for parts of California, Nevada and Arizona.