[2] The county is named in honor of George Mason, a member of the Virginia legislature who campaigned for the adoption of the United States Bill of Rights.

Meltwater from the glacier deposited large amounts of sand in a delta region near at the junction of the Sangamon and Illinois Rivers.

Modern irrigation has made this a highly productive agricultural area.

[4] As of the 2010 United States census, there were 14,666 people, 6,079 households, and 4,060 families living in the county.

[13] Although it voted for the Whig Party in the three elections from 1840 to 1848, Mason County was to be solidly Democratic for the next sixty to seventy years due to its anti-Yankee German-American heritage.

It was not until the 1920 election when bitter resentment was felt by German-Americans at Woodrow Wilson’s postwar policies that Mason supported a GOP candidate.

In the following eighty years, Mason was a Republican-leaning swing county, although isolationist sentiment did cause it to vote narrowly for Wendell Willkie in 1940 and more convincingly for Thomas E. Dewey in 1944.