The first settlement in the Sunnyside area was the Whitmore Cattle Ranch, established in 1879. The Sunnyside name came along when the Denver & Rio Grande Western Railroad established a railhead here and named it for the "sunny side of the Book Cliffs."

Coal was discovered in Whitmore Canyon in 1896 and the Pleasant Valley Coal Company started mining operations in the area in 1898. By 1901, the mining operations were in the hands of the Utah Fuel Coal Company, a subsidiary of the Denver & Rio Grande Western. Utah Fuel operated the mines until 1949 when the mines were sold to Kaiser Steel Corporation. Kaiser Steel was a major player in the area until 1994 when most mining operations came to a halt.