[1][2][3][4] The district is located in the outer zone of Metropolitan Tokyo, roughly 70 kilometres (43 mi) from the center of the capital.

The district was named after the Isumi River, which runs from the mountainous areas of Katsuura to the south of Kujūkuri Beach on the Pacific Ocean.

Tokugawa Ieyasu (1543–1616), as part of the wider unification of Japan, entered the Kantō region in 1590 and awarded the Isumi District to a trusted general Honda Tadakatsu (1548–1610).

In the post Meiji Restoration cadastral reform, the area came under the control of Chiba Prefecture and on April 1, 1889, was divided into two towns (Ōtaki and Asahi) and 21 villages.

Katsuura was elevated to town status on March 12, 1890, Kuniyoshi (future Chōjamachi) on September 22, 1893, Ōhara on December 2, 1899, Onjuku on April 1, 1914, and Okitsu on January 1, 1921.