The Minnesota River flows northeastward along Sibley County's eastern border.

It is fed by the Rush River, whose three branches drain the lower part of the county before merging and then meeting the Minnesota below Henderson.

The county terrain consists of rolling hills etched with drainages and dotted with lakes and ponds, with the area devoted to agriculture.

[6] The terrain slopes to the east and north, with its highest point near its northwest corner at 1,083 ft (330 m) ASL.

[8] Most of the Rush River's watershed is in Sibley County.As of the census of 2000, there were 15,356 people, 5,772 households, and 4,086 families in the county.

Except when voting for Robert La Follette in 1924 and Franklin D. Roosevelt during his two 1930s landslides, Sibley County has been firmly Republican since 1896.

In 1992, Sibley was Ross Perot’s strongest county in Minnesota, losing by only 14 votes to Bill Clinton, whose pluralities in this and the 1996 election are the only Democratic victories in Sibley County since 1940.