The county was named for Johann de Kalb,[4] a German (Bavarian) hero of the American Revolutionary War.

Between 1834 and 1837, after the Potawatomi people had been pushed out, European Americans formed settlements in DeKalb County along streams and wooded areas due to fertile soil, wild game, and food and water opportunities.

Early industries based in DeKalb County included Sandwich Mfg.

Co, Marsh Harvester Co, barbed wire, and Gurler Bros Pure Milk Co.

[6] As of the 2010 United States census, there were 105,160 people, 38,484 households, and 23,781 families residing in the county.

[15] In terms of ancestry, 32.6% were German, 17.5% were Irish, 8.7% were English, 7.0% were Polish, 6.4% were Italian, 6.3% were Swedish, and 4.7% were Norwegian.

[17] As part of Northern Illinois, DeKalb County was a stronghold for the Free Soil Party in its early elections – being among nine Illinois counties to support Martin Van Buren in 1848 – and became overwhelmingly Republican for the century following that party's formation.

The only time it did not back the official GOP nominee between 1856 and 1988 was in 1912 when the Republican Party was mortally divided and Progressive Theodore Roosevelt won almost half the county's vote.

In 1936, Republican nominee Alf Landon won DeKalb County by double digits while losing 46 of 48 states.

In 1992, with the third-party entrance of Ross Perot siphoning votes from Republican George H. W. Bush, Bill Clinton became the first Democrat to carry the county in 140 years.

2016 marked the first time the county had voted Democratic in three consecutive presidential elections since the establishment of the Republican Party.