The Eldorado Mining and Refining Limited company was originally organized in 1927 as Eldorado Gold Mines Limited to develop a gold mine in Manitoba. It's president Gilbert LaBine later found radioactive deposits at Great Bear Lake, Northwest Territories in 1930, which led to the development of the Eldorado Mine at Port Radium. They built a state-of-the-art refinery in Port Hope, Ontario in 1933. Radium production took place between 1933 and 1940 when World War II closed European markets for radium material. The other byproduct of the company was silver, copper, and uranium salts. Uranium was useless until scientists realized the enormous energy potential of the uranium atom. The company reopened the mine at Port Radium in 1942 to supply the United States military with uranium products. The Canadian Government took over the company by purchasing share control in 1943, and in early 1944 the name was changed to Eldorado Mining and Refining Limited.

The crown corporation held a monopoly on uranium prospecting and development in Canada until 1948. Together with the Port Radium deposits, the Eldorado company discovered the Beaverlodge Mine at Uranium City , Saskatchewan. It entered production in 1953. In the 1960s the nature of sales changed when the United States military ceased purchasing of Canadian uranium ores for the purpose of atomic weapons, and from then on uranium was produced for power plants. During this period the name of the company was changed to Eldorado Nucleur Limited. The crown corporation was dismantled in 1988 to become Cameco Limited. The old records of the company are housed in the National Archives of Canada.