He was the United Nations Secretary-General Ban Ki-moon's Special Envoy on Climate Change (2007–08)[1] and for Disaster Risk Reduction and Water (2013–18),[2] Special Advisor to the UN/World Bank High-Level Panel on Water (2016–18), Member of the UN Secretary-General's Advisory Board on Water and Sanitation Agency (UNSGAB, 2007–15), Member of the UN Secretary-General's High-Level Panel for Global Sustainability (GSP, 2010–12), Founding Chair of Global Green Growth Institute (GGGI, 2010–12), Temasek International Panel Member (2004–18) and Independent Non-Executive Director of Standard Chartered plc (2010–2019).

He was a three-term Member of National Assembly of the Republic of Korea (1988–1992, 1996–2004) directly elected by the people of his hometown, Chuncheon.

He has honorary degrees from the Universities of York, Gangwon National, Yonsei, KAIST and Kuala Lumpur.

Han is currently, among others, Chair of High-Level Experts and Leaders Panel on Water and Disasters (HELP, 2007-), Host of the Biennial UN Special Thematic Session on Water and Disasters (2013-), Chair of the Water Advisory Group of Asia Development Bank (2014-), Vice President of Club de Madrid, Member of the WMO Water and Climate Leaders Panel, and Maureen and Mike Mansfield Foundation International Advisory Board Member, Vice-Chair of Zayed Sustainability Prize Jury, Co-Chair of International Finance Forum (China), Chair of the Jury for IFF Global Green Finance Award, Distinguished Professor at Zhejiang University International Business School, Chief International Advisor of International Monetary Institute (IMI) of Renmin University of China, Senior Advisor to Research Institute of ASEAN Economy (Shaoxing, China), and Founder of GG56 Ltd., a big data and blockchain startup.

Han has many publications in both Korean and English, the most recent ones being Here for Global Good: Collected Speeches of Han Seung-soo (YBM, Seoul, 2015) which was translated into Chinese and published by Tsinghua University Press in 2016, Beyond the Shadow of 9/11; A Year at the United Nations General Assembly (SAIS, Washington DC, 2007).