[4][5] The area was home to Native American tribes of the Crow, Blackfoot, Nez Perce, and Sioux.

[4] Walter John Winnett, a rancher who had been adopted into the Sioux tribe, started a ranch in Montana Territory in 1879.

[7] Fort Magginis[8] (built in western Fergus County in 1880) subdued Indian raids in the area, allowing cattle raising to prosper.

Area claims were limited to 320 acres (130 ha); in 1930 many of these lands reverted to the federal government as settlers deserted the town.

This area developed into the Cat Creek Oil Field, producing high-grade crude.

In 1925, the state legislature split the eastern area of Fergus County into a separate entity, named Petroleum County, to denote its status as the first place in Montana where petroleum was discovered, and designated Winnett as its seat.

The southern part of the county consists of moderate hill slopes and gentle valleys, interspersed with steep cliffs.

[9] The northern part of the county has abundant surface water resources, with little agricultural land to use it.

[14] A sandstone ridge with steep dips runs along the northern end of the county.

Sandstones are noted in alternate sequences and are identified at deeper depths in Cat Creek.

[17] Rangeland contains wheatgrass, forbs, shrubs, green needlegrass, blue grama, big sagebrush, plains prickly pear, wooly Indian wheat, weedlike forbs, broom snakeweed, Nuttal saltbush, prairie sandreed, horizontal juniper, plains reedgrass, golden pea, and prairie rose.

Crude oil from the Cat Creek and Rattlesnake Butte fields is piped to refineries in Billings.

Alfalfa and grass hay are grown as cattle feed[20] under irrigated conditions along the main river course and also on the banks of creeks such as Flat Willow, Box Elder, and Macdonald.

Voters in Petroleum County generally vote Republican in national elections.