The Union Fair, started in 1868, began as the efforts of the North Knox Agricultural and Horticultural Society.

At the 2000 census there were 39,618 people, 16,608 households, and 10,728 families living in the county.

There were 21,612 housing units at an average density of 59 per square mile (23/km2).

25.4% were of English, 12.2% Irish, 11.7% United States or American, 7.5% German and 5.7% French ancestry.

At the 2010 census, there were 39,736 people, 17,258 households, and 10,662 families living in the county.

[13] In terms of ancestry, 31.3% were English, 19.0% were Irish, 12.8% were German, 6.9% were Scottish, and 5.9% were American.

[16] In 2012, Knox County voted 55% in favor of a measure to legalize same-sex marriage.

[17] An addition to the Knox County Courthouse designed by Scott Simons Architects was completed in 2005.

It is connected to an annex built in 1977 that is adjacent to the historic, original building that was finished in 1874.