Sheridan County was created by the legislature of the Wyoming Territory on March 9, 1888.

[5] As of the 2000 United States Census,[9] of 2000, there were 26,560 people, 11,167 households, and 7,079 families in the county.

24.8% were of German, 12.3% English, 10.3% Irish, 7.1% American, 6.0% Norwegian and 5.1% Polish ancestry.

30.90% of all households were made up of individuals, and 12.50% had someone living alone who was 65 years of age or older.

As of the 2010 United States Census, there were 29,116 people, 12,360 households, and 7,701 families in the county.

[10] In terms of ancestry, 31.5% were German, 16.3% were Irish, 11.8% were English, 7.5% were American, 6.2% were Norwegian, and 5.1% were Polish.

[20][21] The facility was operated by the Wyoming Board of Charities and Reform until that agency was dissolved as a result of a state constitutional amendment passed in November 1990.

In only one national election since 1948 has the county selected the Democratic Party candidate (as of 2024) which was 1964 when Lyndon Johnson defeated Barry Goldwater by 2.8 percent.

In 2024 Donald Trump received 73.3 percent of the vote; the best ever result for a Republican.