Tonami has a humid continental climate (Köppen Cfa) characterized by mild summers and cold winters with heavy snowfall.

[3] Per Japanese census data,[6] the population of Tonami has recently plateaued after a long period of growth.

The villages of Ota, Minamihannya, Higachi-Nojiri, Yanaze and Nishi-Tonami were annexed on January 15, 1954, followed by Sendan, Sendanyama, Hannya, and Higashi-Hannya on March 1.

The expanded town of Tonami was raised to city status on April 1, 1954.

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