Fremont County was established March 4, 1893, with its county seat in St. Anthony. It was named for John C. Fremont, an explorer known as the "Pathfinder" who passed through the area in 1843. The first settlement was Egin Bench in 1879.
The diversity of the Fremont County landscape reflects its geologic history. Within its borders are mountains, lakes and reservoirs, dense lodgepole pine forests and clearcuts, sagebrush steppes, active sand dunes, irrigated and non-irrigated croplands, and small farming and resort communities.
The southern and western parts of the county lie over the basalt flows of the Snake River Plain - an area of irrigated cropland and sagebrush steppes that also includes a belt of active sand dunes. The county is bordered to the south by the farmlands of Teton, Madison, and Jefferson counties, Idaho, and the west by the rangelands of Clark County, Idaho.
The northern and eastern parts of the county are on the volcanic highlands of the Yellowstone Plateau, where the landscape features lodgepole pine forests, mountain meadows, streams and the headwaters of the Henry's Fork of the Snake River. The county is bordered to the east by the Yellowstone Plateau and the Wyoming border.
The steeply rising Centennial Mountains and Henry's Lake form a distinctive landscape on Fremont County's northern border. The crest of the mountains are both the Continental Divide and the Idaho-Montana state line.
Average annual precipitation in Fremont County ranges from less than 10 inches in the western part of the county to more than 30 inches at higher elevations. The average precipitation can range from 19.11 inches in Ashton, 14.37 inches in St. Anthony and 30.93 at the Island Park Dam.
Elevations in Fremont County range from 4.380 to 10,240 feet. The climate reflects the elevation. The frost-free season of Island Park is about 45 days, compared to about 80 days or more in the Ashton and St. Anthony areas. The mean annual temperature average for St. Anthony area is 42.3 degrees. In the Ashton area the mean annual temperature is 41.1 degrees and in the Island Park area, 36.5 degrees.
The rural county's economy is based mostly on agriculture and recreation or tourism.
As of the 2010 census the county had a population of 13,242.
The county occupies 1,877 square miles or about 1,201,300 acres. Public lands predominate. Only 31.9 percent (599 square miles) of the county's land is in private ownership. About 821 square miles (43.7 percent of the total area) in the northern and eastern portions of the county are in the Targhee National Forest. Another 220 square miles (11.8 percent of the total area), mostly in the western part of the county, is administered by the Bureau of Land Management. The state of Idaho manages about 175 square miles in parcels scattered throughout the county.