Holmes began his diplomatic career by joining the Foreign Service in 1959; his first posting was as a consular and political officer in Yaoundé, Cameroon.

He continued to advance through various State Department positions for the next two decades, including posts in Rome and Paris, until his appointment as Ambassador to Portugal in 1982.

In 1989, he was appointed Ambassador at Large for Burdensharing in which he ensured balanced security responsibility among NATO members, Japan, and other US allies.

[4] During this time his office generated a plan for the Department of Defense to launch new national counterterrorism strategy to respond to "the gauntlet the international terrorists have thrown at our feet."

[5] He is currently an adjunct professor at Georgetown University's School of Foreign Service[6] and a member of Diplomats and Military Commanders for Change.