[2] The county was established in 1913 and named after Franklin D. Richards, an Apostle of the Church of Jesus Christ of Latter-day Saints.

Franklin County is part of the Logan, UT-ID Metropolitan Statistical Area.

Idaho's oldest permanent non-native settlement occurred at Franklin on April 14, 1860, when members of The Church of Jesus Christ of Latter-day Saints led by Thomas S. Smart established the settlement at its present location on the Cub River.

The Bear River Massacre took place in present-day Franklin County on January 29, 1863.

At the 1890 Census, Bingham and Oneida Counties returned five precincts of Dayton, Franklin, Oxford, Preston, and Treasureton with 4,969 residents.

[8] The Oxford and Treasureton precincts were transferred to Bannock County at its establishment in 1893.

[10] In 1953, an aircraft carrying 37 Korean War veterans crashed in the mountains of eastern Franklin County, killing all aboard.

16.00% of all households were made up of individuals, and 8.90% had someone living alone who was 65 years of age or older.

As of the 2010 United States census, there were 12,786 people, 4,079 households, and 3,259 families living in the county.

[21] In terms of ancestry, 34.6% were English, 11.5% were German, 11.0% were Danish, 8.0% were American, and 7.2% were Swedish.

In fact, the last Democrat to win 21 percent of the county's ballots was Jimmy Carter in 1976, and Bill Clinton in 1996 was the last Democrat to gain so much as one-eighth of the county's vote.

In 2016 and 1992 third-party candidates easily outpolled the Democratic candidate, and in the latter case Bo Gritz was only 23 votes shy of forcing Bill Clinton into fourth place.