In 1962, he was elected to the Prince Edward Island House of Assembly and sat in the body for ten years. He served as Minister of Agriculture and Forestry from 1966 to 1972, when he resigned to run in the 1972 federal election. Elected as the Liberal Member of Parliament for Cardigan, he was appointed to the Cabinet of Pierre Trudeau as Minister of Veterans Affairs. He was defeated in the 1979 federal election but returned in the 1980 election and was then reappointed to the Veterans Affairs portfolio. MacDonald reformed veterans' pensions to make them more generous and introduced disability pensions and pensions for prisoners of war.