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.
There was "minor improvement" in California drought conditions over the past week. But as long-term drought persists throughout the west, and storage levels drop, water supply is a worry.
(AP) — The House has waded into a long-running California water war. Lawmakers have endorsed a Republican plan to shift more water to San Joaquin Valley farmers and cut the flow for threatened fish and growers in another part of the state.
The U.S. Drought Center says the past week brought "widespread improvements" in drought conditions in northern California and Nevada.
California's energy grid manager says supply should be adequate for the summer, despite potential natural gas shortages in Southern California.
California water regulators will allow cities and water agencies to set their own conservation targets based on water supply.