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.
The Central Sierra Nevada snowpack this year is larger than the previous four years combined, according to new data from NASA.
It is now the wettest season on record in Northern California, where most of the state gets its water supply.
Some farmers in the San Joaquin Valley will finally get a full supply of water.
California Governor Jerry Brown ended the drought state of emergency in most of California Friday. Water reporting requirements and prohibitions on wasteful practices are still in place.
California is experiencing one of its wettest winters in years. But farmers on the west side of the San Joaquin Valley still won’t receive a full supply of water from the federal Central Valley Project.