It has retained its current configuration since November 3, 1908, when a portion of its previous territory was partitioned to recreate Sheridan County (Sheridan had been dissolved on November 8, 1892, with its territory annexed to McLean).

The southwestern boundary of the county is delineated by the Missouri River as it continues its southeastward flow from the lake.

The county terrain consists of rolling hills, dotted with lakes and ponds in its eastern portion.

[5] The terrain slopes to the south and east, with its highest point on the north boundary line near its NW corner, at 2,201 ft (671 m) ASL.

In terms of ancestry, 54.4% were German, 24.6% were Norwegian, 6.5% were Irish, 5.6% were Swedish, 5.1% were English, and 3.6% were American.

Source:[5] McLean County voters have voted Republican for decades.

In only one national election since 1964 has the county selected the Democratic Party candidate (as of 2024).