He moved to Sydney after leaving school and worked as a clerk and accountant, becoming involved in the labour movement.

[1] He was elected vice-president of the Labor Council of New South Wales in 1910 and the following year succeeded as president, then a rotating office.

[2] During World War I, Duncan supported the efforts of ALP prime minister Billy Hughes to require conscripted soldiers to serve overseas.

Duncan stood as a Nationalist in Granville at the 1917 state election, losing to the incumbent Labor MP and future premier Jack Lang.

[4] In parliament he was "consistent in his emphasis on a White Australia within the Empire but with a unique national character, a Commonwealth government of ample power (seated in Canberra) and effective arbitration in industry.

He supported, where necessary, regulation of business, coupled with high protection, and was suspicious of foreign powers such as the United States".

[1] Duncan's support for the Bruce–Page government declined in his second term and he crossed the floor on a number of occasions.

He and Hughes joined the new United Australia Party (UAP) upon its formation in early 1931, reuniting with their former Nationalist colleagues and dissident Labor MPs led by Joseph Lyons.

[2] Duncan was also a supporter of the New England statehood movement, which sought the separation of northern New South Wales into a new state.

[5] In 1931 he appeared alongside Country Party leader Earle Page at a statehood convention in Armidale, seconding a motion by David Drummond to present a petition to federal parliament for recognition of the new state.

[6] Duncan failed to win UAP preselection for the Senate at the 1931 election,[2] apparently due to the intervention of the Country Party which objected to his inclusion on a joint ticket.

[1] He resigned from the Senate on 1 December 1931,[4] instead standing for the House of Representatives in opposition to Archdale Parkhill, the sitting UAP MP for Warringah.