[3][4] Franklin County is part of the St. Louis, MO-IL Metropolitan Statistical Area and contains some of the city's exurbs.

Occupied by succeeding cultures of indigenous peoples, this area was populated by the historic Osage tribe at the time of European encounter.

The Spanish log fort San Juan del Misuri (1796–1803) was built in present-day Washington.

Among them were the family and followers of Daniel Boone, an explorer from Kentucky who settled the area starting in 1799.

For the next two decades, most settlers came from the Upper South, especially Kentucky and Virginia, bringing their slaves with them to work the land.

In 1833 substantial numbers of German immigrant families began settling in the area, and soon they outnumbered the slave owners in the county.

Former governor and then Confederate General Sterling Price led his cavalry through the county during his Missouri raid of 1864.

Before the war Franklin County had been served by steamboats that moved freight and passenger traffic on the Missouri River.

Bias Vineyard, near the small city of Berger, is located within the Hermann American Viticultural Area (AVA), designated in 1983.

The county is located in the Ozarks region, with steep hills and deep valleys, caves, springs, and sinkholes characteristic of karst areas.

Mining activity in the county included ores of lead, copper, zinc, and deposits of refractory clay.

Manufacturing accounts for the most (23.8%) employment in Franklin County, primarily in the cities of Washington and Union, followed by trade, transportation and utilities (18.8%), education and health care (17.7%), and construction (11.3%).

The biggest employers in Franklin County are the manufacturing firms of Magnet LLC, Cardinal Brands Hazel Division, GDX Automotive, Sporlan Valve Company, Esselte, Silgan, Buddeez, and Meramec Group Inc. as well as the Meramec Valley R-III School District in the public education sector and Schatz Underground Cable Inc. in the construction industry.

Franklin County is divided into four legislative districts in the Missouri House of Representatives,[18] all of which are held by Republicans.

Despite Franklin County's longstanding tradition of supporting socially conservative platforms, voters have advanced some populist causes such as increasing the minimum wage.

The proposition strongly passed every single county in Missouri with 75.94 percent voting in favor as the minimum wage was increased to $6.50 an hour in the state.

Democratic Forty-seventh Vice President Joe Biden won Franklin County with 59 percent of the vote; U.S.

Senator Rick Santorum (R-Pennsylvania) won Franklin County with 60.12 percent of the vote.

Senator Hillary Clinton (D-New York) carried Franklin County with 55.83 percent of the vote.

Senator John Edwards (D-North Carolina) still received 2.96 percent of the vote in Franklin County.

During the 2020 COVID-19 pandemic, Angie Hittson, the director of the Franklin County Public Health Department described being driven to resign from her position by residents who made "daily verbal assaults, threats of violence, and even death threats" against her and her family due to the public-health orders made in response to the pandemic.