[1] The county was named after Bennet Riley, the 7th governor of California, and a Mexican–American War hero.

However, Manhattanites suspected election fraud, and were eventually able to prove that a number of votes were illegally cast.

This tornado is estimated to have been an F4 on the Fujita scale, with a damage path 800 yards (700 m) wide and 100 miles (200 km) long.

Riley County is part of the Manhattan, Kansas Metropolitan Statistical Area.

People aged 15 to 34 years old make up 53.6% of the population of Riley County, one of the highest rates in the United States.

[15] Owing to its history of Yankee anti-slavery settlement in “Bleeding Kansas” days, Riley County became strongly Republican following Kansas statehood, except when over half of its voters supported Progressive Theodore Roosevelt in 1912 who himself was a Republican who had broken away from the party in that election cycle.

Being relatively resistant to the Democratic populism of William Jennings Bryan, Woodrow Wilson and Franklin D. Roosevelt, Riley County stood as the westernmost of thirty-eight US counties to have never voted Democratic for President since the Civil War.

In the 2018 Kansas gubernatorial election, Democratic candidate Laura Kelly won Riley County by a 24-point margin, and in the 2018 US House Election in KS-01, Republican candidate Roger Marshall lost Riley County by a 2-point margin.

In 2024, Donald Trump became the first Republican to win a presidential election without carrying Riley.

[18] The county voted "No" on the 2022 Kansas abortion referendum, an anti-abortion ballot measure, by 68% to 32%, outpacing its support of Joe Biden during the 2020 presidential election.

† means a community is designated a Census-Designated Place (CDP) by the United States Census Bureau.

Among notable current and former residents of Riley County are former Governor John W. Carlin, General Glen Edgerton, millionaire miner Horace A. W. Tabor, NFL receiver Jordy Nelson.