Having achieved the rank of Lieutenant, he served as Landing Force Officer for a ship's company of United States Marines.
After the war, Kubisch spent a year studying at Harvard Business School and then, in 1947, joined the United States Foreign Service.
He was posted to Brazil, and then transferred to Paris in 1949, where he became Assistant to W. Averell Harriman at Marshall Plan Headquarters.
Kubisch resigned from government service in 1950 and spent the next decade working in the private sector as an executive.
In March 1980, he was called out of retirement to serve as U.S. Special Negotiator during the renegotiation of the Treaty of Friendship with Spain.
In retirement, he also served as chairman of the board of the National Defense University; as consultant to the Council on Foreign Relations; and as a director of the Panama Canal Company.