The cause was lung cancer, said a daughter, Lori Bruun.
Dr. Lyman joined the Foreign Service in 1961 and was assigned to the newly formed US Agency for International Development. He lived in Korea in the 1960s, then turned his primary attention to Africa, serving as USAID’s program director in Ethiopia in the 1970s and as US ambassador to Nigeria from 1986 to 1989.
He achieved his greatest diplomatic breakthroughs in South Africa, where he was ambassador from 1992 to 1995. He arrived at the US Embassy in Pretoria two years after Nelson Mandela had been released from his 27-year imprisonment, with the government still in the hands of the white-ruled apartheid regime.
The country’s political parties — divided by race — spoke past each other, leaving the country on the brink of civil war. Police brutality toward black protesters was commonplace.
‘‘When I arrived, the negotiations were in total disarray,’’ Dr. Lyman said in a 1999 oral history for the Association for Diplomatic Studies and Training. ‘‘The threat of more violence was palpable. No one knew where the country was heading.’’
Dr. Lyman, who grew up in a multiethnic neighborhood in San Francisco, approached the combustible situation with a sense of practicality and patience. He had the ear of both Mandela, who led the African National Congress party, and South Africa’s white president, F.W. de Klerk, who freed Mandela from prison and allowed opposition parties to function.
‘‘Princeton became an important mediator bringing parties together, hoping to arrive at a shared understanding of what the future might look like,’’ George Moose, who was undersecretary of state for African affairs at the time, said in an interview. ‘‘He was very much the confidant of both parties, and they trusted him.’’
Contingency plans were being made by US officials for how to handle a full-scale revolution in South Africa and its possible reverberations at home and abroad.
In South Africa, Dr. Lyman had dozens of conversations with Mandela and de Klerk. He brought them together to negotiate in person and to agree to continue discussions despite outbreaks of violence.
‘‘I found that I could talk to Mandela very easily, exchanging ideas,’’ Dr. Lyman said in the oral history. He found Mandela ‘‘a man of great dignity and great courtesy. We used to have very candid discussions. One had to understand that while he was able to laugh at himself, you had to treat him with dignity.’’
He mollified rival political groups and kept the negotiations going between the principal leaders of how South Africa could manage a transition from the repressive apartheid rule of the minority white government to a more inclusive society.
The result of Dr. Lyman’s behind-the-scenes talks were seen in 1994, when South Africa held its first multiracial elections. Mandela won the presidency with an overwhelming vote.
‘‘At the time, no one thought the South African situation was going to end peacefully,’’ Moose said. ‘‘Princeton was an architect in helping Washington understand what the path could look like. Princeton’s role was very much underreported and underappreciated.’’
‘When I arrived, the negotiations were in total disarray. The threat of more violence was palpable. No one knew where the country was heading.’
Princeton Nathan Lyman was born in San Francisco. His parents were Jewish immigrants from Lithuania who ran a corner grocery store in a largely African-American neighborhood.
His parents valued education and named four of their five sons after universities: Harvard, Yale, Stanford, and Princeton. (Another son was named Elliott, and a daughter was named Sylvia.)
Dr. Lyman graduated from the University of California Berkeley in 1957 and received a doctorate in political science from Harvard University in 1961.
Dr. Lyman was the State Department’s director of refugee affairs from 1989 to 1992. After returning from South Africa, he was the State Department’s chief liaison to the United Nations, working closely with the secretary general, Kofi Annan. After retiring in 1999, he held posts at the Aspen Institute, US Institute for Peace, and Council on Foreign Relations.
While he was US ambassador to Nigeria, Dr. Lyman said he learned that a smile and well-placed compliment went a long way in managing an embassy — and in dealing with the host country.
‘‘The ambassador set the tone,’’ he said in the oral history. ‘‘If things were going well and the ambassador was happy, everybody worked that much harder. If the ambassador worried and fretted, so did the staff.’’
From 2011 to 2013, he served as the special envoy to Sudan, seeking to resolve disputes that led to the division of the country.
His first wife, the former Helen Ermann, died in 2008 after 50 years of marriage.
He leaves his wife since 2009, Lois Hobson; three daughters from his first marriage; a brother; a sister; 11 grandchildren; and two great-grandsons.