The rivers and deciduous forests around Shinjo have provided sufficient game and edible plants for human habitation since prehistoric times.

One is a 4,500 year-old clay figure of a woman (45 cm) referred to as the Venus of the Jomon period (縄文時代の女神), excavated in Funagata Town.

During the Yayoi period (400 BC – 300 AD) rice cultivation spread to the area now called Yamagata.

However, near the end of the Heian period, conflict arose and Yamagata was recognized as being under the rule of the Oshu Fujiwara Clan.

The decisive battle of Sekigahara (1600) resulted in the establishment of the Tokugawa Shogunate, the final feudal regime to rule over Japan.

The area formerly administered by the Mogami Clan was divided into regions directly ruled or controlled by feudal lords close to the Tokugawa Shogunate.

Remarkably, the Honma family of Sakata, rich from the shipping business 北船, were for a time the biggest landowners in all Japan.

Part of the stone wall and the moat are all that remain of the castle that was completely destroyed in the Battle of Shinjō during the Boshin War for the Meiji restoration.

From 1878, the area was administered as Shinjō Town within the Mogami District of Yamagata Prefecture and prospered as a center for horse farms providing mounts for Imperial Japanese Army cavalry regiments.

Shinjō has a mayor-council form of government with a directly elected mayor and a unicameral city legislature of 18 members.

In terms of national politics, the city is part of Yamagata District 2 of the lower house of the Diet of Japan.

Held since 1755, the festival was established by the local daimyō (feudal lord) to lift the spirits of the common people after a particularly bad harvest.

The current incarnation of the festival includes traditional dancing, a reenactment of the first "Daimyō Parade", traditional festival vending stalls, and the Yattai Parade, in which each neighborhood in the city constructs large, vivid scenes from Japanese/local history, folklore, and/or fairy tales on wide floats.