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.
Record summer heat has increased fire risk in California and the Western U.S. as drought conditions expand.
Statewide water conservation exceeded the mandatory goal in May and Sacramento reduced water use 40 percent.
In the fourth year of drought in California, sales and use of "safe and sane" fireworks are restricted in some areas, allowed in others.
A California Assembly committee will hear a bill Wednesday that would allow water districts to impose taxes on any business, industry or person who wastes water.
The drought can be blamed for a number of problems and the latest is a major decline in the duck population. A new survey shows lack of rain has led to poor habitats.