During the Edo period, the area was part of the holdings of Tosa Domain ruled by the Yamauchi clan from their seat at Kōchi Castle.

Following the Meiji restoration, the village of Shimatsu (清松村) within Hata District, Kōchi was established with the creation of the modern municipalities system on April 1, 1889.

On August 1, 1954, Shimizu merged with the neighboring towns of Shimokawaguchi, Misaki and Shimokae to form the city of Tosashimizu.

Tosashimizu has a mayor-council form of government with a directly elected mayor and a unicameral city council of 12 members.

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

Tosashimizu is the only city on Shikoku island that hosts a Japan Air Self-Defense Force (ASDF) base.

Sōdabushi, locally called mejikabushi, is one of the main seafood produce in the area with a market share of approximately 70%.

[14][15] On the western shore of Ashizuri peninsula, lies the village of Nakanohama within Tosashimizu, where Nakahama Manjirō was born and became one of the first Japanese to travel to the United States.

Manjirō, a young fisherman, was shipwrecked off the coast of Japan in 1841 and rescued by whaling captain William H. Whitfield of Fairhaven, Massachusetts.

[20] Whale watching is also offered on the east side of the peninsula, based at Kubotsu town to the middle and Shimonokae to the north.

Descending from Itsuta pass (伊豆田峠) toward Tosashimizu, there is Shinnenan (真念庵), a small wayside hermitage 28 km from Kongōfuku-ji.

It was designated to provide a free lodging for pilgrims on that section, where people could also leave their luggage while visiting Kongōfuku-ji, come back to Shinnenan and continue on to the 39th Enkō-ji in Sukumo 50.8 km away.