He studied at the University of Toronto and in Paris and won a silver medal as a student at Osgoode Hall Law School.

As Vice-Chairman of the Board of Control, Duncan charged the city with being run as a family compact and accused the Toronto Transit Commission of not disclosing its books.

[2] Duncan ran as a federal Liberal candidate for the House of Commons of Canada in the 1940 election in the Danforth electoral district.

Duncan then joined the Co-operative Commonwealth Federation (Ontario Section), and ran again for Mayor of Toronto in 1944 but was defeated by Frederick J. Conboy by 20,000 votes.

He also ran in the 1945 Ontario election as the CCF's candidate in the riding of High Park against George Drew, accusing him of being financed by "beer barons", but was defeated.