Union County is part of the Sioux City, IA–NE–SD Metropolitan Statistical Area.

The Progressive Farmer rated Union County second in the "2006 Best Place to Live Survey" in the U.S., because "its schools are good, its towns neat and its people friendly".

The Brule Creek flows southeasterly across the central part of the county, emptying into the Big Sioux.

The county terrain consists of rolling hills, devoted to agriculture except around built-up areas.

[3] The terrain slopes to the south and east; its highest point is near its northwest corner, at 1,509 ft (460 m) ASL.

In terms of ancestry, 42.0% were German, 18.2% were Norwegian, 17.7% were Irish, 7.9% were English, 6.8% were Swedish, and 3.7% were American.