In 1945 Cals became leader of the Catholic People's Party in the municipal council of Nijmegen until 1946. He was elected as a Member of the House of Representatives in 1948. From 15 March 1950 to 2 September 1952 he was State Secretary for Education, Arts and Sciences, serving from 15 March 1950 until 2 September 1952 in the Drees-Van Schaik and Drees I cabinets. He became Minister of Education, Arts and Sciences serving from 2 September 1952 until 24 July 1963 in the cabinets Drees II and III, Beel II and De Quay, he helped pass the "Mammoetwet", a law that transformed secondary education. In the debate, he spoke for 6 hours and 50 minutes, setting a record. In 1963, however, he returned to the House of Representatives. Alongside his duties there, he was a member of the board of governors of the University of Groningen, chairman of the Arts Council and a member of the Press Council.