The 20th century saw other mineral development come to the eastern prairies. Coal had been mined in Montana since the early days of settlement, but large-scale exploitation of the incredible reserves of fossil fuels waited until the railroads arrived. The Northern Pacific developed Red Lodge, and later Colstrip, as sources of fuel for its steam trains, and the Milwaukee built up Roundup as its source. After trains were converted to electricity, coal mining ceased for several years. However, as machinery and technology refined the techniques of strip mining, the vast reserves of coal in the Fort Union Formation in southeastern Montana became more attractive.

In the 1970s, energy companies proposed building four electric generators in Colstrip, with the power to be sold to markets on the West Coast. Battles erupted in courtrooms and communities as the breadth of the mining and environmental damage became clear. The issues surrounding development sundered many communities as the benefits of conservation and economic opportunity were debated. Despite grassroots opposition from ranchers, Indians, and environmentalists, the generators went in.

Oil and gas exploration also brought wealth to some eastern Montana communities. Refineries helped Billings boom during the 1970s, and towns such as Sidney, Broadus, and Baker escaped the worst of recent agricultural downturns because of the presence of large nearby oil reserves. Plans are in place to build a new wind- and coal-fired generating plant between Jordan and Circle.