William B. Edmondson, the U.S. ambassador to South Africa during a period of high tension between the white-minority South African government and the United States, died Dec. 5 at a medical center in Prescott, Ariz. He was 86.

The cause was cardiac arrest, said his son, Paul Edmondson.

A career Foreign Service officer, Mr. Edmondson served as ambassador to South Africa from 1978 to 1981, a time of unrest and upheaval after a black uprising in Soweto and the 1977 death of anti-apartheid activist Steve Biko while in police custody.

As the U.S. ambassador, Mr. Edmondson delivered the Carter administration’s message of condemnation of apartheid. He reinforced U.S. support for civil rights and reforms that would lead to majority rule by black South Africans.

William Brockway Edmondson was born in St. Joseph, Mo. He served in the Army during World War II. He was a 1950 graduate of the University of Nebraska, and he received a master’s degree from the Fletcher School of Law and Diplomacy at Tufts University in Medford, Mass., in 1951.

He began his Foreign Service career in 1951 and had early assignments in Africa and Switzerland. He was deputy assistant secretary for African affairs in Washington before his appointment as ambassador to South Africa. He retired in 1986 after serving as a senior inspector and then deputy inspector general of the State Department.

In retirement, Mr. Edmondson was president of DACOR (Diplomatic and Consular Officers Retired) and was a volunteer guide at the Library of Congress.

In 2008, he moved from Arlington County to Arizona.

Survivors include his wife of 62 years, Donna Kiechel Edmondson of Prescott; two children, Barbara Schneider of Prescott and Paul Edmondson of Washington; and four grandchildren.

— Bart Barnes