The county was named after Mount Jefferson, the second highest peak in Oregon with an elevation of 10,497 feet, which marks the county's western skyline. The county is bounded on the north by Wasco County, on the east by Wheeler and Crook Counties, on the south by Deschutes County, and on the west by Linn and Marion Counties. The county encompasses 1791 square miles. Madras, named after the city in India, was incorporated in 1911 and serves as the county seat. A new county courthouse was built in 1961.
County government is administered by a three-member board of commissioners.
Political jurisdictions in the county include Senate District 27, Representative District 55, and the Second Congressional District. The county's population at its first federal census in 1920 was 3,211. Principle industries are agriculture, forest products, and recreation. The fertile North Unit Irrigation District in the central part of the county produces seed, potatoes, hay, and mint. The eastern part of the county has dry wheat farming and grazing land for cattle, and the western part is timber country.