[2] Stephenson County is included in the Freeport, IL Micropolitan Statistical Area, which is also included in the Rockford-Freeport-Rochelle, IL Combined Statistical Area.

The county was named for Colonel Benjamin Stephenson, an official of the Illinois Territory.

[6] As of the 2010 United States census, there were 47,711 people, 19,845 households, and 13,015 families residing in the county.

There were 22,081 housing units at an average density of 39.1 per square mile (15.1/km2).

[12] In terms of ancestry, 46.6% were German, 12.5% were Irish, 10.0% were English, and 8.0% were American.