Mamiya was founded in May 1940 by Mamiya Seiichi and Sugawara Tsunejirō as Mamiya Kōki Seisakusho, Mamiya Optical Works). It was based in Tokyo, Hongo, and its first camera was the Mamiya Six, a 6×6 folder with coupled rangefinder that focused by moving the film plane. There were many versions in the Mamiya Six series, and it was the only Mamiya model for eight years. Despite the onset of World War II a year after the release of the first Mamiya Six model, the company continued to thrive, expanding production facilities in February 1942 and again in February 1944, employing 150 staff. By March 1944, a secondary factory was opened at Tokyo University for the manufacture and assembly of lenses. The company closed the facility in Tokyo in March 1945 and relocated. As early as October 1945, a month after Japanese surrender, Mamiya was the first Japanese company to receive a substantial order from the Central Purchasing Office of SCAP, which allowed the company to resume full-scale production in January 1946 at new facilities in Tokyo.

To ensure a reliable supply of lenses, Mamiya bought an optical company at Setagaya, Tokyo. Mamiya began to make its own Stamina shutters and Neocon ('new con[struction]) lenses in 1947 at the Setagaya plant. In 1950, the Setagaya plant became Setagaya Kōki K.K., making shutters and lenses. The name Sekor that appears on most Mamiya lenses comes from Setagaya Kōki, with r probably for renzu, lens). Setagaya Kōki was merged into Mamiya Kōki in 1963.