Early career

• since 1985: Member of the Social Democratic Party of Germany
• since 1993: Member of the executive of the SPD district of Weser-Ems
• 1996–2006: Member of Hinte Municipal Council (2001–2006: Council chairman)
• since 2001: Member of the SPD executive
• since 2002: Chairman of the SPD district of Weser-Ems

Member of the European Parliament, 2002–2005

In the European Parliament, Duin served on the Committee on Regional Policy, Transport and Tourism (2000–2004) and on the Committee on Industry, Research and Energy (2004–2005). In addition to his committee assignments, he was a member of the parliament's delegation for relations with Switzerland, Iceland and Norway from 2000 until 2004.

Member of the German Bundestag, 2005–2012

Duin was elected as Member of the German Bundestag in the 2005 federal elections; he got 58 percent of the vote in the Aurich-Emden region in northern Germany. In parliament, he served on the Committee on Economic Affairs and Technology. Within his parliamentary group, he held the position of commissioner for industry policy (2006-2009) and chairman of the working group on economic affairs and technology (2009-2012).

In addition to his committee assignments, Duin served as deputy chairman of the German-British Parliamentary Friendship Group from 2010 until 2012. He was also a member of the German-Austrian Parliamentary Friendship Group, the German-Dutch Parliamentary Friendship Group and the German-Swiss Parliamentary Friendship Group.

Also in 2005, Duin was elected chairman of the SPD in Lower Saxony, succeeding Wolfgang Jüttner. He resigned from this position in 2010 and was replaced by Olaf Lies. Within his party, he belongs to the Seeheim Circle.

Together with Doris Ahnen, Bärbel Diekmann, Barbara Ludwig and Heiko Maas, Duin co-chaired the SPD's 2009 national convention in Dresden.^[1]

State Minister for Economic Affairs, 2012–2017

Duin resigned as Member of the Bundestag on 21 June 2012 and was appointed Minister for Economics, Energy, Industry and Commerce of the state of North Rhine-Westphalia in the government of Minister-President Hannelore Kraft on the same day.

In the negotiations to form a Grand Coalition following the 2013 federal elections, Duin was part of the SPD delegation in the working group on economic policy, led by Ilse Aigner and Hubertus Heil.

During his time in office, Duin was a member of the German-Russian Friendship Group set up by the German Bundesrat and the Russian Federation Council. From 2014 and 2016, he was one of the members of Germany's temporary National Commission on the Disposal of Radioactive Waste.^[2]

Corporate boards

• Stadtwerke Köln, Chairman of the supervisory board (since 2018)^[5]
• NRW.BANK, Ex-Officio Member of the supervisory board (2012-2017)

Non-profit organizations

• Friedrich Ebert Foundation (FES), Member^[6]
• Forum for Future Energies, Member of the Board of Trustees
• Christoph Metzelder Foundation, Member of the Board of Trustees
• Stiftung Lebendige Stadt, Member of the Foundation Board
• Norddeutscher Rundfunk, Member of the Broadcasting Board
• German United Services Trade Union (ver.di), Member
• Workers' Welfare Association (AWO), Member
• Eurosolar, Member