Kempthorne's first major political victory was in 1986 when he became the mayor of the city of Boise. He served in that position until 1993, when he became a member of the United States Senate for Idaho. He had been elected in 1992 to replace another Republican, Steven Simms, who was retiring. Kempthorne served one term, not running for reelection in 1998. Instead he ran for governor of Idaho, winning the Republican nomination as a supporter of the retiring Republican governor, Phil Batt.
Kempthorne, who was notable for conducting his campaign while still being active in Washington as a senator, won the election in a landslide, receiving 68% of the vote while his Democratic opponent, Robert Huntley received 29%. He was reelected in 2002 with 56% of the vote, compared to his Democratic opponent, Jerry Brady who polled 42%. Kempthorne is notable for his conservative views, particularly on economic issues.