During the Second World War, he served with the Royal Canadian Navy. He joined the Department of External Affairs in 1945. He was a senior public servant in the Departments of Trade and Commerce and Industry, Trade and Commerce from 1958 to 1971. (Assistant deputy minister until 1964, he served as deputy minister thereafter.) Warren was also appointed to the General Agreement on Tariffs and Trade (GATT) for several years in the 1960s.
He resumed his diplomatic career in 1971, serving as high commissioner to the United Kingdom until 1974. He was ambassador to the United States from 1975 to 1977. An appointment as Canada’s ambassador and coordinator at the Tokyo Round of the GATT world trade negotiations followed from 1977 to 1979. (See also Diplomatic and Consular Representations.)
Warren was vice-chairman of the board of the Bank of Montreal from 1979 to 1986. He then served as deputy North American chairman of the Trilateral Commission from 1986 to 1990. He was the principal trade advisor to the province of Quebec during the negotiation of the North American Free Trade Agreement.
Warren was awarded the Order of Canada in 1982.