Cohagen was named for the maiden name of the first postmaster, Harry Harris' mother. The Harrises were the first settlers in the area, with the post office established in their home, on the old LU Ranch in 1905. By 1915 Cohagen was booming with three stores, a schoolhouse, a restaurant, two hotels, a doctor and a dentist. By 1948, most of the buildings were gone and the population had decreased. Cohagen was known for being good sheep country but in recent years as many as 5,000 were killed due to a single winter storm. Oftentimes teams of horses pulled snowplows to clear land so the sheep could graze. The latest census figures shows the population at about eight.
Cohagen is located in eastern area of Montana, southeast of Jordan on Highway 59.