The Montana Power Company was incorporated on December 12, 1912, as a result of a merger of the Butte Electric and Power Company, the Madison River Power Company, the United Missouri River Power Company, and the Billings and Eastern Montana Power Company. The merger was brought about by John D. Ryan, head of the Anaconda Copper Mining Company, and John G. Morony, president of the First National Bank of Great Falls. Each of the constituent companies had been formed as the result of mergers of earlier companies.

The Butte Electric and Power Company, was formed in 1901 to take over the assets and liabilities of the Butte Lighting and Power Company, which was, in turn, the successor to several local Butte power generating plants, including the Brush Electric Light and Power Company, the Silver Bow Electric Light Company, the Butte Electric Light and Power Company, the Silver Bow Electric Light and Power Company, the Butte General Electric Company, the Phoenix Electric Company, the Butte Gas Light and Coke Company, and others. The Butte Electric and Power Company's proerties also included a hydro-electric power station on the Big Hole River from The Montana Power Transmission Company, successor to the Big Hole Improvement Company; and a majority interest in several Great Falls companies including the Great Falls Street Railway Company, the Boston and Great Falls Electric and Power Company, and the Great Falls Electric Properties.

The Missouri River Electric and Power Company, was organized in 1911, as a successor to the United Missouri River Power Company (UMRPC), which had gone into receivership. The UMRPC was itself a merger of the Missouri River Power Company, the Capital City Power Company, and the Helena Power Transmission Company. By 1912 the successor company owned most of the hydro-electric facilities on the Missouri River, including the first Canyon Ferry Dam completed in 1898 by the Helena Water and Electric Power Company; the Hauser Dam built in 1907 by the Helena Power Transmission Company; and the Holter Dam, begun around 1907 by the Capital City Power Company; and, due to financial difficulties, not completed until after the merger into Montana Power Company.

The Madison River Power Company was organized in 1905 by Butte Electric and Power Company interests to take over the Nunn power plant on the Madison River from The Power Company. The Madison River Power Company also constructed the Lower Madison Development in 1906 and transmission lines to Butte. In addition the Company acquired the Bozeman Electric Light Company, the Bozeman Street Railway Company, the Gallatin Light, Power and Railway Company, and the Livingston Water Power Company.

The Billings and Eastern Montana Power Company was organized in 1908 to take over various power developments along the Yellowstone River, including the Yegan Brothers properties, the Yellowstone River Power Company, the Billings Water Power Company, the Montana Trading Company, and the Big Timber Electric Light and Power Company.

After the organization of the Montana Power Company in 1912, the company continued to consolidate power developments around the state. In 1929 several major systems were acquired. The Missoula Public Service Company, was a merger of the Missoula Light and Water Company and the Missoula Street Railway Company both developed by William A. Clark. The various Helena properties which had been consolidated under the Helena Gas and Electric Company in 1927, also became part of Montana Power Company in 1929. A third major acquisition in 1929 was the Thompson Falls Power Company, including its predecessor the Northwestern Development Company. The Great Falls Townsite Company, the Great Falls Power Company, and the Great Falls Street Railway Company were all acquired by Montana Power Company between 1931 and 1936. The last major system acquired before World War II was the Union Electric Company of Dillon, which included the properties of its predecessors the Rife Electric Company and the Dillon Electric Light and Power Company. In addition, many local utilities were added for which there are no records in this collection.

For more comprehensive information on the history of the predecessor companies see "Corporate Growth" (Box 1, Folder 2) and Douglas F. Leighton, The Corporate history of the Montana Power Company, 1882-1913, (M.A. Thesis. Montana State University, Missoula, 1951).