The Powder River flows northward through eastern Johnson County.

Froom 1889 to 1893, Johnson County was the scene of the Johnson County War, where wealthy cattle ranchers seeking to control limited resources enlisted hired guns to fight smaller settlers and homesteaders.

In 1911, the boundaries of Johnson County and adjacent Crook, Natrona and Weston Counties were adjusted to run along federal land survey lines.

[6] At the 2000 United States Census,[10] there were 7,075 people, 2,959 households and 2,006 families in the county.

28.50% of all households were made up of individuals, and 12.00% had someone living alone who was 65 years of age or older.

At the 2010 United States Census, there were 8,569 people, 3,782 households, and 2,410 families in the county.

[11] In terms of ancestry, 31.6% were German, 22.4% were Irish, 18.3% were English, and 6.1% were American.

Since Wyoming statehood, the voters of this county have selected the Democratic Party candidate in only three national elections: William Jennings Bryan in 1896; Woodrow Wilson in 1912 by two votes and with only 37.5 percent of the total vote (due to Roosevelt's independent run that year); and Franklin D. Roosevelt by seventy votes in his 1932 landslide.

In the 1964 Democratic landslide it was Barry Goldwater's best county in Wyoming, and second-best in the West behind Utah's traditional banner Republican county of Kane.

[18][19] The Wyoming Board of Charities and Reform operated the facility until the agency was dissolved as a result of a state constitutional amendment passed in November 1990.