[1] Its county seat is Rock Island; its largest city is neighboring Moline.

[4] The Rock River (which the Sauk and Meskwaki peoples called Sinnissippi, meaning "rocky waters")[5] flows from Whiteside County and points further east and north and joins the Mississippi River at Rock Island.

The Sinnissippi Mounds, dating from the Hopewell period and on the National Register of Historic Places are upriver at Sterling in Whiteside County.

[7] As of the 2010 United States census, there were 147,546 people, 61,303 households, and 38,384 families residing in the county.

[16] At one time Mississippi Valley Airlines had its headquarters in Quad City Airport in the county.

From 1932 on, it has consistently backed Democratic Party presidential candidates, except for the national Republican landslides of 1952, 1956, 1972, and 1980.