[2] The county was established in 1869 and named after Fort Lemhi (or Limhi), a remote Mormon missionary settlement from 1855 to 1858 in Bannock and Shoshone territory.

Habitation of the Lemhi and Salmon Rivers dates back 14,000 years ago.

The Salmon River was the dividing point among the first cultural split which occurred after 5,000 BC.

[5] The Lemhi band of Shoshoni gained early access to horses and had developed into a migratory culture.

The Lemhi band was forced to a reservation on February 12, 1875, even though the tribe failed to ratify the treaty creating it in 1868.

Meriwether Lewis and three other members of the expedition were the first Americans of European descent to enter what is now Idaho.

Within a month, travel down the Salmon and Snake Rivers was ruled out and the expedition headed for Lolo Pass on the Bitterroot Range.

[7] Michael Bourdon of the Hudson's Bay Company established the Lemhi Valley as a base of trapping operations in 1822.

Finnan McDonald, Alexander Ross, David Skene Ogden, Jedediah Smith, and Jim Bridger all spent time in the area.

A stagecoach route was established from Montana in May 1867 to the present location of Salmon City.

Additional mining operations occurred at Yellow Jacket in 1869, Gibbonsville in 1877, Gilmore in 1880, Blackbird in 1892, and Leadore in 1904.

Boise County was created in 1863, with its northern boundary running just north of the ridge dividing Birch Creek) from the Lemhi River.

The following session, the legislature passed the bill again, officially creating Lemhi County on January 9, 1869.

[17] The western and southern county boundaries for Lemhi were problematic and adjusted frequently.

At its creation, Lemhi inherited Idaho County's southern and eastern boundaries, which effectively created two areas of noncontiguous territory, with one of the sections containing territory along the Salt Lake to Virginia City stagecoach route just north of the present town of Humphrey, Idaho.

With legislative approval, voters in that territory voted to become part of Fremont County in 1896, finalizing the southern and eastern boundary.

The boundary was moved westward to the divide between the Payette and Salmon Rivers in 1873, moved eastward to a line drawn through the confluence of the Middle Fork and main Salmon rivers in 1885, and then mistakenly extended to the northern line of Idaho County in 1903 before being finalized at its present location in 1911.

Custer County was partitioned from Lemhi in 1881 with its initial boundary excluding the Loon Creek area.

As of the 2010 United States Census, there were 7,936 people, 3,576 households, and 2,267 families living in the county.