Merrill served as counselor to the Undersecretary of Defense for Policy from 1981 to 1983; as a member of the Defense Policy Board from 1983 to 1990 and again from 2001 to 2003; and as Assistant Secretary General for Defence Support at NATO Headquarters in Brussels from 1990 to 1992 under President George H. W. Bush. He was appointed to chairman of the Export-Import Bank of the United States by George W. Bush, serving from 2002 to 2005. He represented the United States in negotiations on the Law of the Sea Conference, the International Telecommunications Union and various disarmament and exchange agreements with the former Soviet Union. For many years he chaired the White House Fellow Commissions regional panels. Merrill also served on President George H. W. Bush's Gulf War Air Power Survey and President Reagan's Commission on Cost Control in the Federal Government.