[2] Per Japanese census data,[3] the population of Iwanuma has recently plateued after a long period of growth.

The area of present-day Iwanuma was part of ancient Mutsu Province, and the Takekoma Inari Shrine claims to have been founded in 842 AD.

The area came under the control of the Date clan of Sendai Domain during the Edo period, under the Tokugawa shogunate.

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

In terms of national politics, the city is part of Miyagi 3rd district of the lower house of the Diet of Japan.