The county was formed by uniting parts of two territorial counties, Day and Woodward and named for Albert H. Ellis, a Vice President of the Oklahoma Constitutional Convention.
          In 1892, the south part of Ellis County was a part of County E, which was named Day County at the first election after the Cheyenne and Arapaho Opening in 1892. The County Seat was located at Ioland and then moved to Grand on Nov 13, 1893. 
          The Oklahoma Constitutional Convention decreed that Day County should cease to exist on Nov 16, 1907, and a new county, to be named Ellis should be formed. 
          The part of Day County south of the South Canadian River became a part of Roger Mills County - that part north of the river became a part of the new Ellis County. In addition, Townships 20 - 24 N Ranges 23 - 26 W, previously included in Woodward County, would now be part of Ellis County. 
          Grand was named the temporary county seat, but was changed to Arnett by vote of the people and moved on Oct 1, 1908.

Built in 1912, the historic Ellis County Courthouse stands in the center of the Courthouse Square in Arnett. Just south of the Courthouse stands the old jail and an historic log cabin that is still in use today by the local Boy Scouts. The Courthouse Square is the center of town activities and festivals throughout the year.