The county seat was initially named "Taylorsville" in honor of Colonel James P. Taylor (it was changed to "Mountain City" in the 1880s).

[5] The county sent a sizable delegation to the Greeneville session of the pro-Union East Tennessee Convention in June 1861.

[6] Due in large part to the county's remoteness, the railroads did not reach Johnson until the early 20th century.

The arrival of the railroads during this period aided the development of the timber and manganese mining industries.

The highest place in Johnson County is Snake Mountain's lower peak, (near the North Carolina state line), at 5,518 feet (1,682 m).

Male inmates in the Northeast Correctional Complex, southwest of Mountain City, account for 1,299 (7.4%) of the county's population.

[17] Johnson County is a long-term Republican stronghold and is located within Tennessee's 1st congressional district, which has not been represented by a Democrat since 1881.

Since a Republican Party presidential nominee first appeared on the ballot in Tennessee in 1868, there has only been one occasion when Johnson County's voters didn't vote for the official Republican Party candidate, and that was in 1912, when voters voted for the official Bull Moose Progressive Party candidate, Theodore Roosevelt, the former Republican president of the United States from 1901 to 1909.

Steve Earle's song "Copperhead Road" is about a family of moonshiners from Johnson County – where, until 2018, alcohol was prohibited ever since the Twenty-First Amendment.

The childhood home of Valene Ewing, a character in the TV series Dallas and Knots Landing, is in Johnson County, in the fictional community of Shula.