He was the last remaining survivor of the historic Rivonia Trial, which sentenced activists including Nelson Mandela to life imprisonment.
Mr Mlangeni had been admitted to hospital in the capital Pretoria, with abdominal pains.
South African President Cyril Ramaphosa paid tribute, saying his death signified “the end of a generational history”.
“With his passing as the last remaining Rivonia triallist, Bab’Mlangeni has indeed passed the baton to his compatriots to build the South Africa he fought to liberate and to reconstruct during our democratic dispensation,” Mr Ramaphosa said.
Mr Mlangeni spent 27 years in prison alongside Mr Mandela, Dennis Goldberg, Walter Sisulu and other activists who were sentenced for planning to overthrow the apartheid government.
After his release, he served as a legislator in South Africa’s first democratic parliament from 1994.
In his later years, he was chairman of the integrity committee of the ruling African National Congress party, which was responsible for investigating corruption allegations against its leaders.
“His passing sounds the last post on a courageous generation of South Africans who forfeited their freedom, careers, family lives and health so that we could all be free,” the foundation of former South African archbishop Desmond Tutu and his wife Leah said in a statement.
They also recalled Mr Mlangeni’s comment: “I did not go to prison for 26 years for people to steal from the poor.”
