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.
Mandatory statewide water conservation rules have ended in California. But Sacramento-area users conserved 22 percent in June, compared to June 2013.
California and federal agencies say a new strategy is needed to save the endangered Delta smelt.
California is in the fifth consecutive year of drought, but state water managers ended mandatory conservation rules. Local water suppliers now determine conservation rates, and some have low or no targets. A water expert says that's 'shortsighted.'
California is in the fifth consecutive year of drought. Although mandatory statewide water conservation is over, the State Water Resources Control Board says water conservation remains a "top priority."
The California Water Resources Control Board Wednesday says Californians cut water use by 28 percent in the final month of mandatory statewide conservation.