John Mark Slack Jr. (March 18, 1915 – March 17, 1980) was an American politician from West Virginia. He was a Democrat. Slack was born in Charleston, where he attended the public schools. He later studied at the Virginia Military Institute in Lexington, Virginia. 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.