Crapo was born in Idaho. He went to several universities, including Brigham Young University and the Harvard University school of law. He served as clerk to an appeals court judge and then returned to Idaho to become a lawyer. While practicing law in Moscow, Idaho in the 1980s, he was active in the Republican Party's campaigns for seats in the state legislature. He was elected to the state senate himself in 1984.
Crapo was a member of the state senate from 1985 until 1992, when he was elected to the United States House of Representatives. He was a member of the house from 1993 until 1999, being reelected in 1994 and 1996. He was elected to the United States senate in 1998.