Cyril Ramaphosa, President of South Africa, addressed the nation with a message of condolence on the death of Nelson Mandela's anti-apartheid activist and personal lawyer, George Bizos, who died at the age of 92.
Bizos died peacefully at his home of natural causes, the family said.
Bizos, who came to South Africa as a 13-year-old fleeing the Nazi occupation of Greece, played a key role in the legal fight to end apartheid, the racist system used to oppress South Africa's Black majority for decades.
"This is a very sad moment for our country," President Cyril Ramaphosa said.
"He contributed immensely, immensely, to the attainment of our democracy" and was one of the architects of the post-apartheid constitution.
"We dip our heads in honour."
The soft-spoken but determined Bizos represented Mandela from his treason trial in 1964 until the former South African president's death in 2013.
Bizos is credited with getting Mandela to add the words "if needs be" to his speech from the dock in which he said he was prepared to die for his ideals.
The addition was seen as an escape clause, avoiding any impression that Mandela was goading the court to impose the death penalty.
Mandela, who called Bizos "a man who combined a sympathetic nature with an incisive mind," trusted him not only to represent him in court but later to carry messages from African National Congress leaders imprisoned on Robben Island to their colleagues in exile.
After Mandela's release, Bizos lent his legal expertise to negotiations that led to South Africa's first all-race elections in 1994.
Mandela became the country's first Black president.