Cass represented the riding of Grenville—Dundas just south of Ottawa. He first won a seat in the 1955 provincial election and was brought into the provincial cabinet in 1958. He served 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. While he was Attorney-General, Cass continuted to practice on the weekends as a small-town lawyer in his hometown of Chesterville. However, to avoid a conflict of interest, he stayed out of the courts and practised solely as a solicitor.
Cass provoked serious controversy when, in response to an organized crime scare, he proposed Bill 99, a sweeping amendment to the Police Act, which would have broadened police powers allowing the Ontario Police Commission the right to interrogate and cross-examine witnesses in camera in contravention of the traditions established by English Common Law. When queried on the amendments by the press, Cass said "Yes, these are drastic, draconian measures that in some ways are really unbelievable in a country that has an English common law system." The resultant uproar and, in particular, a speech by Liberal MPP Andy Thompson forced Cass to resign from Cabinet on March 23 1964. Thompson's success buoyed him into the position of leader of the Liberal Party several months later.
Following his re-election in the 1967 general election, Cass was rehabilitated by being chosen Speaker of the legislature once the body reconvened on February 14 1968. He was a popular presiding officer and served until his retirement at the 1971 election. As Speaker, Cass ruled that Opposition MPPs could no longer begin their questions during Question Period with the word "why", as questions were "usually not aimed at soliciting information from cabinet ministers, but at giving the questioner an opening to catalogue his complaints about the government." This resulted in various verbal gymnastics as Opposition MPPs strugged to find a way to say "why" without saying "why."