There are 12 US Post Offices in Umatilla County serving a total of 75,869 residents equating to 6,322 residents per post office. It's estimated that approximately 121,095 packages are handled across the 12 county post offices annually. Umatilla County /\u02ccju\u02d0m\u0259\u02c8t\u026al\u0259/ is a county located in the U.S. state of Oregon. As of the 2010 census, the population was 75,889. The county seat is Pendleton, but the largest city is Hermiston. The county is named for the Umatilla River. Umatilla County is part of the Pendleton\u2013Hermiston, OR Micropolitan Statistical Area. It is included in the eight-county definition of Eastern Oregon."