Resident Population in Jefferson County, MT was 13.04800 Thous. of Persons in January of 2023, according to the United States Federal Reserve. Historically, Resident Population in Jefferson County, MT reached a record high of 13.04800 in January of 2023 and a record low of 5.23800 in January of 1970. Trading Economics provides the current actual value, an historical data chart and related indicators for Resident Population in Jefferson County, MT - last updated from the United States Federal Reserve on May of 2024.