Thomas is an expert on agriculture and rural development. He has served in important positions in several state agencies. His most lengthy job was as executive vice president of the Wyoming Farm Bureau from 1965 until 1974. He was also an independent small businessman. In 1984 he was elected to the state legislature, in which he served until 1989.
In 1989, Dick Cheney, who occupied Wyomings only seat in the United States House of Representatives, resigned from it in order to become secretary of defense. Thomas became the republican candidate to replace him in the April 1989 special election, and he won. He was reelected in 1990 and 1992. In 1994 he ran for the United States Senate and won that election as well. He was reelected in 2000.