Dave Thomas was appointed district attorney for Jefferson and Gilpin counties by Governor Dick Lamm in 1985. He lost his election bid for the office in 1986.
In 1992, he ran again for district attorney and voters elected him to a four year term. He was re-elected in 1996 and 2000.
Facing term limits, Thomas ran against fellow Democrat Mike Feeley in 2002 for his party's nomination to the new 7th Congressional District and lost.