Rainfall is significant throughout the year, but heavier during the rainy season in June and July, and also in September, when a large number of typhoons hit Japan.

During the Kamakura period much of the area became part of a vast shōen landed estate controlled by the Kujō family of aristocrats from Kyoto.

During the Onin War, the kanpaku Ichijō Norifusa fled Kyoto in 1468 and settled in Nakamura, in what is now Shimanto.

[11] In this year, the head of the Nakamura fief was punished for the offence of turning down a post on the Tokugawa Junior Council.

[11] The Nakamura area lost a great deal of wealth and independence, and became a minor rural region.

[11] Following the Meiji restoration, the village of Nakamura was established within Hata District, Kōchi with the creation of the modern municipalities system on October 1, 1889, and was raised to town status on November 10, 1898.

Shimanto has a mayor-council form of government with a directly elected mayor and a unicameral city council of 20 members.

In terms of national politics, the city is part of Kōchi 2nd district of the lower house of the Diet of Japan.