Mr. Wellenstein was born in The Hague, Netherlands, on 15 November 1949. He holds a baccalaureate from the European School in Luxemburg and a degree in Economics from the University of Groningen which he obtained in 1975.

In 1976, after his military service, Mr. Wellenstein entered the Ministry of Economic Affairs, serving in the Directorate-General for Foreign Economic Relations, initially dealing with export financing and export subsidiary matters. From 1982 to 1985, he was Private Secretary to the Secretary of State for Foreign Trade. After a sabbatical leave at the Fletcher School of Law and Diplomacy (USA) in 1986 he was appointed Head of Division for Bilateral Economic Affairs with Asia, Africa and the Middle East. Then, in 1987, he moved to the Directorate for Multilateral Affairs as Head of the GATT Division. From 1989 to 1992, Mr. Wellenstein was posted to the European Directorate as Director of European Affairs, and was involved in the Maastricht Treaty negotiations.

In 1992, he was named Director for GATT, UNCTAD, Investment Agreements and Commodity Agreements within the Directorate for Trade Policy, and was involved in the Uruguay Round negotiation eventually leading to the creation of the WTO. In 1994, Mr. Wellenstein moved to the Ministry of Defence, where he served as Head of the Directorate-General for Budget and Management. His responsibilities included budget planning and financial management, strategic guidance and information management. Then, in 2002, he joined the Ministry of Foreign Affairs as Head of the International Criminal Court task force, holding the post of Permanent Representative of the Netherlands to the ICC.

From 2007, until his nomination as Permanent Representative of the Netherlands to the OECD, Mr. Wellenstein served as a member of the Change Team at the Ministry of Foreign Affairs.