Umatilla County was created on September 27, 1862, out of a portion of Wasco County.  Adjustments were made to the
 county's boundaries following the creation of Grant, Morrow, Union, and Wallowa Counties. This legislative act also
 designated Marshall Station as the temporary county seat. An 1865 election selected Umatilla City, now known as Umatilla,
 as the county seat. With the development of wheat farming, population shifted to the north and east parts of the county, and a
 subsequent election in 1868 moved the county seat again to Pendleton.

 The Umatilla Indian Reservation was established by the Treaty of Walla Walla in 1855. The Umatillas, Walla Wallas, and
 Cayuse tribes were resettled there, and is located immediately southeast of Pendleton.