Adams County was formed out of Whitman County on November 28, 1883.
From Ritzville's designation as county seat in 1884 until 1892, courts were briefly held in rented space before the county purchased a building from N.H. Greene in 1885. In 1891 the commission bonded $20,000 in order to erect a permanent court house. The building was built by the Spokane firm of Burnham & Clapp and completed by August 20, 1891 when it was accepted by the county. It was a two story brick building with a four story tower and a stone basement. A two story addition was added to the front in 1905 with a new four story tower giving the courthouse a whole new appearance. This courthouse was replaced by the current art-deco one in 1941.
According to the U.S. Census Bureau, the county has a total area of 1,930 square miles (4,998 km²), of which, 1,925 square miles (4,986 km²) of it is land and 5 square miles (12 km²) of it (0.25%) is water.
Adams County is a historically Republican county. It was one of only three counties in the state to be won by Barry Goldwater in 1964. Along with Lincoln County it is the only county in Washington that has not been won by a Democratic candidate in last 48 years. The last Democratic candidate to win the county was Franklin D. Roosevelt in 1936. The Republican candidate has won by more than 13% in every Presidential election since 1992 and George W. Bush won the county by over 40% both times he ran.
In state elections the results are similarly Republican. In the last thirteen elections no Democratic candidate for governor has carried Adams County. The last Democratic candidate for US Senate to win the county was Henry M. Jackson in 1982.
In the 2008 elections John McCain received 66.32% of the county's vote. Meanwhite Republican nominee for governor Dino Rossi received 69.83% of the county's vote. They also gave Republican representative Cathy McMorris Rodgers 82.45% of their vote. In other results Republican Sam Reed received 74.6% of the county's vote in his run for re-election as Washington Secretary of State; Republican Allan Martin received 67.25% of the vote for state treasurer; and Republican Rob McKenna received 76.28% in his re-election run for Attorney general. All county positions, where party are mentioned, also went to the Republican.