[2] During the late 1990s Duncan served as a policy analyst for BP's political affairs team, where he worked on the company's strategy for emerging economic prospects in post-communist eastern Europe and the former Soviet Union.

From 2004 to 2005 he acted as the head of policy & communication for the Scottish Refugee Council, a charity which offers advice to those taking asylum within Scotland.

He resigned from his position in 2013 to pursue candidacy for the upcoming European elections with the Scottish Conservatives, following Struan Stevenson's announcement that he would not seek re-election.

[12] Duncan was selected by the Scottish Conservatives as their candidate for Perth and North Perthshire in the 2017 UK general election.

[13] After he failed to win the Perth constituency,[14] the Prime Minister's Office announced in June 2017 that Duncan would be granted a life peerage and thus become a member of the House of Lords, in order to take up his appointment as Parliamentary Under-Secretary of State for Scotland and for Wales.

He is the honorary president of English Speaking Union Scotland, previously serving as Chairman (2014–2017) and Speech & Debates Officer.

He retains links to academia, acting as an advisor to University of St Andrews' Institute of Legal and Constitutional Research.