Siebert was appointed United States Ambassador to Sweden in 1994 by President Bill Clinton and served in this capacity until 1998.
Ambassador Siebert served as parliamentarian and negotiator on such contentious issues as Palestinian membership in the ITU.
Ambassador Siebert also serves on the Board of Advisers of the International Criminal Court (ICC) Project which operates within the American Bar Association's (ABA) Center for Human Rights.
Ambassador Siebert is Chairman of the Stockholm International Peace Research Institute – North America (SIPRI-NA) headquartered in Washington, DC.
He lives with his wife, Debbie, in Washington, D.C., and is the father of their four children: Sarah, Lauren, Thomas Siebert II (Teddy), and Trevor.