Nicholas Haysom, a South African lawyer who fought against apartheid and later became a leading peace negotiator for the United Nations, died on March 17 in Manhattan at the age of 73. His daughter said he had been suffering from heart and lung problems. The secretary general of the United Nations praised his lifelong commitment to justice, dialogue and reconciliation in some of the world’s most fragile regions.

Haysom was repeatedly jailed in the 1970s and 1980s for his anti-apartheid activism, including months in solitary confinement. A founder of a law firm that supported Black trade unions, he later helped draft South Africa’s post-apartheid constitution and served as chief legal adviser to President Nelson Mandela from 1994 to 1999.

With Mandela’s support, Haysom moved into international mediation, helping negotiate peace agreements in Burundi and Sudan and contributing to the process that led to South Sudan’s independence. He joined the United Nations in 2005 and held senior roles in Iraq, Afghanistan, Somalia and South Sudan, where he was serving at the time of his death. Colleagues remembered him as a trusted and skilled mediator who sought to ensure that minority voices were heard in deeply divided societies.