Since her appointment as minister of minerals and energy in 1999, Phumzile Mlambo-Ngcuka has consistently won praise as one of the most effective ministers in the South African government.
By Justin Pearce
BBC News, Johannesburg
Mining, for which she has carried political responsibility, remains a key sector within the South African economy, but also one that has been urgently in need of reform.
Mlambo-Ngcuka has won praise for her role in government
The minister is admired for having steered the industry successfully through a time of change, increasing black South African participation in the sector without alienating foreign investors.
Her achievements in government have been acknowledged with her promotion to the deputy presidency.
As deputy president, she moves into the highest office ever occupied by a woman in South Africa, and her appointment is the latest step in President Thabo Mbeki's efforts to increase female representation in government.
It is only in the past few months that two incidents in particular have cast a shadow over her reputation.
When environmental activists called for an investigation into the illness of workers at a nuclear installation, Ms Mlambo-Ngcuka responded by saying that South Africa needed laws to prevent organisations from "spreading panic" - a remark that angered defenders of free speech.
More recently, it emerged that her brother had benefited from a loan involving state money that was to have been used in a government oil transaction.
Bulelani Ngcuka started the investigation which eventually led to Zuma's downfall
The minister's office strongly denied any suggestions of wrong-doing, saying the loan had been repaid before the matter was made public.
But any discontentment with her appointment is less likely to hinge on anything she has done than on the man to whom she is married.
Her husband, Bulelani Ngcuka, is the former chief prosecutor who initiated an investigation into the financial affairs of sacked former Deputy President Jacob Zuma - the man she replaces.
In 2003, he angered Mr Zuma by publicly announcing that while there was a prima facie case against the then deputy president, there was insufficient evidence to convict him.
Her appointment to the deputy presidency will inevitably leave Ms Mlambo-Ngcuka open to criticism from Mr Zuma's supporters who already believe - albeit without concrete evidence - that her husband was part of a conspiracy to get rid of the former deputy president.
Born in 1955, Phumzile Mlambo began her career as a schoolteacher, before moving into development work.
Working both inside and outside South Africa during the apartheid years, she rose to political prominence after the unbanning of the ANC in 1990.
She entered government as a deputy minister in 1996, and has headed the Minerals and Energy Ministry since Thabo Mbeki was elected president in 1999.
Her appointment as deputy president does not automatically mean she will succeed Thabo Mbeki, but it certainly improves her chances of becoming Africa's first elected female head of state.