Coffin served in the Pacific Theater with the United States Navy as an ensign and later as a lieutenant from 1943-1946. He was admitted to the bar and commenced the practice of law in Lewiston, Maine and was later a law clerk for Federal judge, district of Maine, 1947-1949.

Coffin served as chairman of the Maine Democratic State committee from 1954-1956 and was elected as a Democrat to the Eighty-fifth and Eighty-sixth Congresses (January 3, 1957-January 3, 1961). He was not a candidate for renomination in 1960, but was unsuccessful for election as Governor of Maine.

Coffin was managing director of Development Loan Fund until October 1961 when he became deputy administrator of the Agency for International Development and served until 1964. He was appointed to serve as United States Representative to Development Assistance Committee of the Organization for Economic Cooperation and Development in Paris, France from 1964-1965.

Coffin was appointed to the United States Court of Appeals for the First Circuit on October 2, 1965 and served as chief judge of that court from 1972-1983. He assumed senior status on February 1, 1989. Coffin was the chairman of the U.S. Judicial Conference Committee on the Judicial Branch from 1984-1990. He is a resident of South Portland, Maine.