In January 1977 Armacost was selected as a member of the National Security Council to handle East Asian and Chinese affairs under the Carter administration until July 1978, when he was replaced by Nicholas Platt. Years later he was appointed to be the United States Ambassador to Japan from 1989 to 1993, Under Secretary of State for Political Affairs from 1984 to 1989, and United States Ambassador to the Philippines from 1982 to 1984, during a critical period of political upheaval during the Ferdinand Marcos presidency.
He served as Acting Secretary of State from January 20, 1989 to January 25, 1989. In the interval between 1995 and 2002, Armacost served as president of Washington D.C's Brookings Institution, the nation's oldest think tank and a leader in research on politics, government, international affairs, economics, and public policy.
He has received the President's Distinguished Service Award, the Defense Department's Distinguished Civilian Service Award, and the Secretary of State's Distinguished Services Award.
Armacost is the author of three books, the most recent of which, Friends or Rivals?, was published in 1996 and draws on his tenure as ambassador. He also co-edited, with Daniel Okimoto, The Future of America's Alliances in Northeast Asia, published in 2004 by Shorenstein APARC. Armacost has served on numerous corporate and nonprofit boards, including TRW, AFLAC, Applied Materials, USEC, Inc., Cargill, Inc, Carleton College, and The Asia Foundation.
Armacost received a Bachelor of Arts in international relations from Carleton College in 1958, an honorary degree  in 1989, where he was chairman of the board of trustees from 2004 to 2008, and earned his Ph.D. from Columbia University.
- The Mismatch between Northeast Asian Change and American Distractions, by Michael H. Armacost, NBR Analysis, January 2007
- Appearances on C-SPAN
- Forging Even Closer Japan-US Ties | Nippon.com
- "Inside the Manila Embassy". Kai Bird. Archived from the original on October 9, 2009. Retrieved 12 Nov 2009.
- Ministry of Foreign Affairs (MOFA): 2007 Autumn Conferment of Decorations on Foreign Nationals, p. 1.
| U.S. Ambassador to Japan
Richard W. Murphy
| U.S. Ambassador to Philippines
Stephen W. Bosworth
|Non-profit organization positions|
| President of the Brookings Institution
1995 – 2002