NorthWestern Corporation owns NorthWestern Energy, a utility company that serves South Dakota, Nebraska, and Montana that is based in Sioux Falls. As of December 31, 2007, the company serves approximately 650,000 customers. The company's corporate headquarters are located in Sioux Falls while the headquarters for the South Dakota operations (and which was the headquarters for the old NorthWestern Public Service Company) are in Huron, SD.

The Montana operations (formerly known as the Montana Power Company) were acquired around 2000 after that state passed some legislation allowing the electric utility industry to be 'unbundled'. Out of state investors (led by Pennsylvania Power & Light) then immediately acquired the generation assets of Montana Power, leaving that part of the territory vulnerable to high "spot" prices on the energy market (creating some chronic discontent that led the Montana Public Service Commission to submit the one "no" vote that effectively nullified a potential sale of NorthWestern to Babcock & Brown Infrastructure of Australia). In 2014, NorthWestern purchased the dams that were originally part of Montana Power for $900 Million from the PPL Corporation.

In March, 2003, a class action was filed against Northwestern Corp, claiming that defendants artificially inflated the stock price through material omissions and materially false and misleading statements. A $41 million settlement fund was established in 2005. On April 1, 2003, as a result of the ongoing evaluation and review of certain accounting entries, NorthWestern said it expects to restate prior unaudited quarterly results for the first three quarters of 2002.

In March 2009, NorthWestern Energy was involved with a gas explosion in Bozeman, Montana. The explosion destroyed several businesses and killed one person. By November, the company was sued by numerous owners of the destroyed businesses.