Slack's public service career began in 1948, when he became a member of Kanawha County Court, where he sat until 1952. He subsequently served as Kanawha County Assessor from 1952 to 1958.
When three-term 6th district Representative Robert Byrd decided against seeking re-election in order to run for the U.S. Senate, Slack won the Democratic nomination and was easily elected that November. He went on to be re-elected 11 times, never facing serious opposition. His district was renumbered as the 3rd District in 1963, after West Virginia lost a district as a result of the 1960 Census. Slack voted in favor of the Civil Rights Acts of 1960,[1]1964,[2] and 1968,[3] and the Voting Rights Act of 1965.[4]
Slack served until his death of a heart attack in Alexandria, Virginia on March 17, 1980.[5] He was replaced by John G. Hutchinson, then mayor of Charleston, after a special election, who served until the end of the term. He was interred in Cunningham Memorial Park, St. Albans, West Virginia.