Besides his service as Chair and Dean, Thiemens has been active in external service:
* Board of directors, San Diego State University Research Foundation, 2006-2009
* City of San Diego Science Advisory Board (2002-2005)
* San Diego Natural History Museum Board of Trustees (2001-2006)
* San Diego Chamber of Commerce Environmental Advisory Board 1998-1999.
* ECO AID Board of Advisors (1999-2002)
* Science Advisory Board. Office of Trade and Business Development. San Diego (2002)
* Kyoto Prize Symposium San Diego organizing committee, UCSD Lead. 2006-2016.
* Council, The Meteoritical Society, 2008-2011.
* Committee on the Significance of International Transport of Air Pollutants (2008-2009) National Research Council. (Global Sources of Local Pollution Report) 
* Understanding the Impact of Selling the Helium Reserve (2008-2009). National Research Council (Selling the Nations Helium Reserve Report) National Research Council
* Planetary Protection Committee. Mars Sample Return (2008-2009). National Research Council (Assessment of Planetary Protection for Mars Sample Return Mission)
* Committee for Planetary Protection Standards for Icy Bodies in the Outer Solar System (2011) National Research Council
* Board on Energy and Environmental Systems 2009-2016. National Academy of Sciences.
* Searching for Life Across Space and Time. (2016-2017). Space Science Board Requested study.
* Space Sciences Board (2014–present). National Academy of Sciences
* Executive committee, Space Sciences Board (2018—present) National Academy of Sciences.
* Associate editor, Proceedings National Academy of Sciences, 2007 to present. National Academy of Sciences