Mlambo-Ngcuka, 57, was deputy president of South Africa under Thabo Mbeki between 2005 and 2008. Bachelet stepped down in March from U.N. Women, a body for gender equality and the empowerment of women created in 2010.
"Phumzile Mlambo-Ngcuka brings to this position a wealth of experience in advocating for women's issues, with a combination of strategic leadership, consensus building and hands-on management experience," U.N. spokesman Martin Nesirky said.