Career highlights include earthquake relief in Guatemala, liaison with human rights groups in the former Soviet Union, negotiation of drug control and status of forces agreements in Honduras.

Ambassador to Mongolia (1993–1996), Johnson had traveled to Ulaanbaatar before the U.S. had established diplomatic relations.

Between 1997 and 1999 he was one of three members of the Independent International Commission on Decommissioning during the Northern Ireland peace process.

While serving on the Commission, he led the team that carried out the very first voluntary decommissioning of paramilitary weapons in Northern Ireland, which saw the destruction of submachine guns, handguns, ammunition and improvised explosives.

In addition, Cape Verde became only the sixth country in Africa to achieve Category 1 status for its civil aviation, and in July 2005, Cape Verde’s airline began direct flights to the U.S. From the fall of 2005 until June 2006, Ambassador Johnson worked in the U.S. Mission to the Organization of American States, leading the U.S. team in the negotiations for a Social Charter of the Americas.