In 2008, Lehmann was appointed inaugural holder of the UNESCO Chair for Sustainable Urban Development in Asia and the Pacific — a recognition of the international significance of his research and its influence on regional policy and sustainable urbanisation across the Asia-Pacific.

He has an ability to lead high-level strategic initiatives across multidisciplinary and international environments and provide strategic advisory insight across future city and urban transformation initiatives.

As a member of the United Nations Economic and Social Commission for Asia and the Pacific (UN ESCAP) Expert Group on the New Urban Agenda (2012–2016), he co-authored the UN-Habitat report State of Asian and Pacific Cities 2015. He also served on the UN working party for HABITAT III preparations (2013–2016), contributing to global sustainable urbanisation frameworks and the formulation of the Sustainable Development Goals (SDGs) for 2015–2030.