[3] Jefferson County is part of the Idaho Falls, ID Metropolitan Statistical Area.

Stops were established at Market Lake (Roberts), Sand Hole (Hamer), and Camas.

Small settlements grew up around the stagecoach stops with the most significant development occurring at Market Lake.

The Camas and Market Lake precincts are the first to emerge in census data under Bingham County in 1890.

At that point, 379 residents were enumerated in the Camas precinct and Market Lake counted 218.

[10] A common characteristic of Jefferson County's early settlements was their reliance upon a series of canals to deliver water from the Snake River.

The first of these systems commenced at Menan in 1880 with construction of the Long Island Canal.

The Owlsey Canal in western Jefferson County had its origins April 13, 1909.

Mud Lake became the defining point for Oneida County's boundary with Alturas County in 1877 with a portion of the western boundary changed to a line drawn north from a point on the Snake River due south of the "sink of Camas Creek".

[12] The change transferred settlements at Camas and Market Lake to Oneida County.

As of the 2010 United States Census, there were 26,140 people, 8,146 households, and 6,698 families living in the county.

[21] In terms of ancestry, 31.4% were English, 13.9% were German, 8.2% were American, 5.9% were Danish, and 5.3% were Irish.

The last Democrat who carried the county in a presidential election was Harry S. Truman in 1948.

[25] The last Democratic candidate to receive more than twenty percent of the county's vote was Jimmy Carter in 1976.