[3] The county was created in 1865[4] and named for President Thomas Jefferson.

[5] Jefferson County is part of the Helena, MT Micropolitan Statistical Area.

[13] As of the 2010 census, there were 11,406 people, 4,512 households, and 3,301 families residing in the county.

There were 5,055 housing units at an average density of 3.1 per square mile (1.2/km2).

In terms of ancestry, 31.3% were German, 19.1% were Irish, 17.2% were English, 10.8% were Norwegian, and 4.2% were American.