From 1990 to 1998, he served as chief negotiator for the Swedish Ministry of Environment. In this capacity, he led the Swedish delegation through preparations for the Rio Conference in 1992, the Conference itself, and its aftermath. He also led Swedish delegations to UN climate change negotiations from the beginning until his retirement in October 2001. During this time, he co-chaired several international negotiating groups. He also chaired the EU Working Party on Climate during the Swedish EU Presidency in 2001. He continues to serve as an adviser to the Ministry of Environment.

Ambassador Kjellén has also served as chairman of the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (2001-2004); co-chairman of the Steering Committee for the European Climate Platform (2005-2011); and co-chairman of the Advisory Committee for the European Capacity Building Initiative (since 2005).

He has received four honorary doctorates: from Silsoe College, Cranfield University, UK (1997, science); Gothenburg University (1999, philosophy); Mälardalen University (2005, technology), and Stockholm University (2011, law).