From The Idaho Encyclopedia...

    Fremont County was the first county created after Idaho was admitted to the Union. It was organized by the Legislature in 1893 from the northern part of Bingham County. It was named for John C. Fremont, the pathfinder who surveyed the West for the Government.
The area included about 6,082 square miles north of the present Bonneville County boundaries, excepting the northeastern part of Lemhi County. In 1895, the northeastern boundary line of Lemhi County was placed farther west, fixing it at its present position, and the area was annexed to Fremont. In 1903 Fremont County was made larger still, with the annexation of the Big Hole Mountain area.
In 1913, Madison and Jefferson counties were created, taking the southern half of Fremont County. In 1919 Clark County was created, taking the northwestern corner.
The first settlement in what is now Fremont County was made in 1879 on Egin Bench, half-mile west of the Fort Henry site on the north side of the river. This settlement, the first in the county within the area of the North Fork of the Snake River, consisted of 11 log cabins. It was the settlers of Egin Bench who named St. Anthony Falls because of its similarity to the falls by that name in the Mississippi River. In 1887, a bridge was built across the river at the present site of the town of St. Anthony. Two years later the townsite of St. Anthony was platted.
Lake, which was a camping ground for the Indians on Henry's Lake, was for many years a point on a stage line which passed through the northern part of the county. Rea, a city in Shotgun Valley, was one of the oldest post offices in the county and the center of a stock-raising area.
    About 1880, agricultural communities were established, most of them by Mormon people who filed claims and transformed the area into farms. Among the first of these communities were Wilford, Teton, Parker and Marysville. Ashton, the last important point in Idaho on the highway leading to Yellowstone National Park, is one of the newer towns of eastern Idaho.