Duncan has been Chief Executive of Friends of the Earth Scotland since 2003 and has led the organisation in successful campaigns against nuclear power in Scotland, for a tough Climate Change Act and for improved waste recycling. In 2008 he was surprised to be named among the 50 most influential people in Scotland by the Scotsman newspaper.

He has been an environmental campaigner for twenty years. Amongst other roles, he is also a board member of Changeworks and the Scottish Environmental Law Centre, and advises the Joint Research Councils on energy research. He is also a member of the UK Green Fiscal Commission and of the editorial board of the Local Environment journal.

He lives in Edinburgh with his partner and two young children who remind him daily of the importance of campaigning for a sustainable future.