Tancredo earned a bachelor's degree in political science from the University of Northern Colorado in 1968, and he worked as a junior high school civics teacher before entering politics. In 1976 he was elected to the Colorado House of Representatives, where he served two terms. In 1981 Tancredo was named a regional representative for the U.S. Department of Education, a post he held into the early 1990s.

In 1993 Tancredo returned to the private sector as the president of a libertarian think tank. He remained there until he launched his campaign for the U.S. House of Representatives in 1998. He entered Congress in 1999 on a platform that emphasized border control, budget reduction, and immigration reform; his views on security and border issues are outlined in his book In Mortal Danger: The Battle for America's Border and Security (2006). He was also an opponent of abortion rights and gun-control legislation. He focused on many of these same issues in his campaign for the presidency in 2008. Although his tough stance on immigration resonated with many in the Republican Party, his poll numbers never exceeded single digits, and in December 2007 he withdrew from the race.