Cass represented the riding of Grenville—Dundas just south of Ottawa. He first won a seat in the 1955 provincial election. He was appointed to provincial cabinet in 1958 as Minister of Highways under Leslie Frost and became known as the cabinet's troubleshooter. In 1961, Frost retired and Cass was considered one of his potential successors. Instead, he supported provincial treasurer James Allan at the party's leadership convention. Allan was defeated by John Robarts, who appointed Cass Minister of Municipal Affairs, and later, Attorney General in 1962.