Located entirely in Millard County, it connects the city of Fillmore with Flowell and the ghost town of McCornick over a distance of 16.9 miles (27.2 km).

The route begins as 400 North at SR-99 (Main Street) in Fillmore.

After 4.6 miles (7.4 km), the route turns north and continues through rural Millard County for about 12.3 miles (19.8 km) until it ends at US-50.

SR-100 was originally established in 1935 as connecting Fillmore at SR-1 (Main Street) and Flowell.

[2] In 1969, the 12.6 miles (20.3 km) from near Flowell northward to US-50 was transferred to the route, resulting in its current length and alignment.