Tagajō is in the coastal plains of east-central Miyagi Prefecture, bordered by the Pacific Ocean to the east.

The area of present-day Tagajō was part of ancient Mutsu Province, and has been settled since at least the Jōmon period by the Emishi people.

Per the Shoku Nihongi, following a huge earthquake in the year 715 AD, a large number of people migrated to this area from the southern Kantō region, forming numerous fortified settlements in the hinterland.

After the start of the Meiji period, the modern village of Tagajō was established with the creation of the municipalities system on April 1, 1889.

[6] Tagajō 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 Miyagi 4th district of the lower house of the Diet of Japan.