In 1966 Williams was elected to the Montana House of Representatives in District 23 of Silver Bow County, winning reelection in 1968. During the years 1969–1971 he served as the executive assistant to Montana Representative John Melcher. Williams was a member of the Governor's Employment and Training Council from 1972 to 1978 and served on the Montana Reapportionment Commission from 1972 to 1973. In 1974 Williams ran an unsuccessful primary election campaign against future Senator Max Baucus for the Democratic Party nomination for Montana's U.S. House 1st District Representative. Baucus would win the November elections defeating RepublicanDick Shoup. In 1978 Williams ran a successful primary campaign against Dorothy Bradley to win the Democratic nomination for the 1st District of Montana. In November Williams defeated Republican Jim Waltermire in one of Montana's largest door-to-door campaigns, and winning 57% percent of the vote, getting elected to the 96th U.S. Congress.
Re-elections
In 1980 Williams won reelection against Jack McDonald with 61% of the vote; in 1982 against Bob Davies with 60%; in 1984 against Gary Carlson with 67%; in 1986 against Don Allen with 62%, 1988 against Jim Fenlason with 61%; in 1990 against Brad Johnson. In 1992 Montana lost its second seat in the U.S. House of Representatives, leaving Williams to campaign against fellow incumbent Ron Marlenee. Williams narrowly won with 50% of the vote. In 1994 he was elected to his ninth and final term, defeating Cy Jamison with 49% the votes. He chose not to run for reelection in 1996, and Republican Rick Hill defeated Bill Yellowtail to become Montana's new U.S. Representative that year. As of 2020, Williams is the last Democrat to represent Montana in the U.S. House.
Recent history
Williams is Senior Fellow and Regional Policy associate at the Center for the Rocky Mountain West and he serves on the Boards of Directors for the National Association of Governing Boards of Universities and Colleges, the National Association of Job Corps, and The President's Advisory Commission for Tribal Colleges. Williams was on the board of directors of the Student Loan Marketing Association, the now disbanded GSE subsidiary of U.S.A. Education. Williams also writes newspaper columns on occasion. Nominated for a seat on the Montana Board of Regents of Higher Education in 2012 by then-governor Brian Schweitzer, Williams endured opposition to his pending confirmation. It arose due to publication of an out-of-context statement made to a New York Times reporter regarding a half dozen players on the University of Montana football team who recently ran afoul of the law. He referred just to those six as "thugs," but his statement was taken as referring to the entire team and program. The cause for confusion was due to Williams' continued attempts to clarify his statements. He was first quoted by ESPN saying, "Montana recruits thugs". His clarification to a half dozen players did not come until his confirmation hearing. By that point the damage had been done. His confirmation to the Board of Regents was blasted to the Senate floor and the Republican-majority Senate rejected his appointment. Williams is a member of the ReFormers Caucus of Issue One.
