Dewar was first elected to the Legislative Assembly of Prince Edward Island as a PC candidate representing district of 2nd Prince in the 1955 elections, going on to represent the riding for 21 years. He served as Minister of Education from 1959 to 1966 and Provincial Secretary from 1964 to 1966. Dewar ran unsuccessfully for a seat in the House of Commons in 1984. He also served as chairman of the O'Leary Museum and Library Association, later the Prince Edward Island Potato Museum.