An ANC statement announced the death and said Ms. Duarte had been on medical leave undergoing cancer treatment.
Ms. Duarte began her career in accounting in the 1970s and became involved in organizing women’s groups that served as a grass-roots network for the ANC and the United Democratic Front, both of which were outlawed by South African authorities during the apartheid era.
She was detained without trial in 1988 and released under “restriction orders,” which placed controls on where someone could travel or whom they could meet with. She then became a core member of Mandela’s team after he was freed in 1990 after 27 years in prison — part of a political process that led to the end of White rule with Mandela’s election as president in 1994.
She served as Mandela’s special assistant until the election and later modeled herself as an uncompromising — and, to critics, sometimes mercurial — defender of the ANC and Mandela’s vision after his death in 2013 at 95.
Admirers embraced her as a fiery stalwart from the anti-apartheid struggle and one of the few consequential women in South African leadership. She also could be a political liability at times, flying into rages over uncomfortable questions about alleged misdeeds by the ANC or within her own family.
She reserved a special venom for journalists. In one outburst at a 2019 news conference, she unleashed a finger-wagging screed against a South African reporter who sought her response to allegations of kickbacks and financial abuses by the ANC’s then-secretary general, Elias “Ace” Magashule, while he was premier of South Africa’s Free State province from 2009 to 2018. (The trial has been set for September pending Magashule’s appeal to have the case thrown out.)
She also pushed back against journalists’ inquiries about alleged links between her estranged husband, politician John Duarte, and powerful tycoon brothers Atul, Rajesh and Ajay Gupta, accused of siphoning off huge sums from state entities during the tenure of President Jacob Zuma from 2009 to 2018. Zuma was ordered back to jail last year after a medical furlough from custody was revoked; he is appealing the ruling.
In a rare moment of contrition, Ms. Duarte in 2018 said her son-in-law Ian Whitley “made a mistake” by being fast-tracked for a position with South Africa’s Finance Ministry in 2015 during the four-day tenure of then-finance minister Des van Rooyen, who also has been under investigation for possible ties to the Gupta business empire.
“Welcome to the alternate world of Jessie Duarte,” columnist Kalim Rajab wrote in South Africa’s Daily Maverick in April 2019. “A world in which journalists must [toe] the line and not ask why she tolerates venal corruption.”
At the same time, however, Ms. Duarte’s reputation grew among supporters, championing Mandela’s ideal of the ANC as a big-tent movement open to all. Ms. Duarte came from a family of Indian and Muslim lineage, and in 2019, she broke ranks with ANC leadership to denounce the party as “tribalistic and racist” for, in her view, snubbing those not from full Black heritage.
Many South African women also took pride in Ms. Duarte’s past as a coordinator of women’s resistance during the apartheid era and her continued ability to slug it out in the boys’ club of South African politics. Over the decades, she used her influence in the ANC to press for changes such as paid maternity leave and government child-support aid.
“Even in the democratic era, Jessie was relentless in advancing the position of women in all areas of public and private life,” South African President Cyril Ramaphosa said in a eulogy. “She confronted the patriarchal attitudes and practices that sought to diminish the role and contribution of women in parliament, in government and across society.”
Yasmin Dangor, who went by Jessie for much of her life, was born in Johannesburg on Sept. 19, 1953, and she was one of nine siblings in a home where Afrikaans was the main language.
After Mandela was freed from the notorious Robben Island prison, Ms. Duarte served as special adviser to him and to prominent anti-apartheid activist Walter Sisulu until the 1994 election. She stepped down from a post overseeing security in 1998 after acknowledging that she drove a state car without a driver’s license.
She later held top ANC positions and was South Africa’s ambassador to Mozambique from 1999 to 2003. She returned to take on various ANC roles, including spokeswoman. Until she left for medical reasons late last year, Ms. Duarte was the ANC’s acting secretary general.
She had two children with John Duarte. Information on survivors was not immediately clear. Her brother, Achmat Dangor, a South African poet and author, died in 2020.
As an ANC insider, Ms. Duarte was often asked for her perspective on Mandela, whom she called by his clan name, Madiba. She recounted his disappointment at sharing the 1993 Nobel Peace Prize with South Africa’s last apartheid president, F.W. de Klerk, and she marveled at his decision two years later to use a rugby victory to help unite the country.
In a 2015 interview with the PBS show “Frontline,” Ms. Duarte recounted watching the finals of the Rugby World Cup in 1995, won by South Africa over New Zealand in what became a critical moment of national unity for Mandela’s government.
“I am radical. I am sort of very left,” she said. “When the ANC was unbanned, people like myself were the hard core, rough, tough. We weren’t going to take any kind of concessioning for White people who had given us a hard time.”
She was astonished to see Mandela put on a South African jersey to honor the victorious Springboks, a team beloved by many Whites in South Africa but viewed by others as a hated symbol of apartheid.
“Madiba saw symbols as an important issue, and sport is such a symbol,” Ms. Duarte said. “Madiba understood that if you were going to get people to build a nation, you didn’t leave any element of that nation unattended. … The one time I did feel emotional is when I saw him in that [jersey]. I thought wow, this is what patriotic symbolism is about. It’s giving people that sense of … you can have a difference politically, but we come together as a nation when we win things. We don’t win it as a single individual. We win it as a group, as a collective.”
Lesley Wroughton in Cape Town contributed to this report.