When the anti-apartheid movement was banned in 1960 and the leadership resolved to adopt the armed struggle, Mlangeni was one of the six leaders who were chosen for military training.
By 1963, the ANC high command had established sabotage units nationwide, operating from a farmhouse with outbuildings on Liliesleaf Farm in Rivonia, northern Johannesburg. The property belonged to Arthur Goldreich, a painter and costume designer for the King Kong jazz opera.
Here the leadership operated underground under assumed names to evade the security police. Meetings were held on Monday evenings.
The plan was that they would be visible in their communities during the day to allay suspicions while in actual fact they were underground operatives.
Mlangeni travelled widely in his Zephyr Zodiac, recruiting new members to the ANC and to explain the movement's activities. During these travels he disguised himself by wearing a large beard and introducing himself to authorities as Reverend Mokete Mokoena.
But the special branch of the police was closing in and on the night of July 11 1963, they raided Liliesleaf and the houses of leaders such as Mlangeni, arresting 19 men.
On June 12 1964 the accused were sentenced to life imprisonment after they were found guilty of treason.
Mlangeni later remarked that he was not surprised by the sentence: "I expected to be arrested one day. But I was already steeled physically and mentally. I was one of those referred to as amadelakufa - hardened and fearless freedom fighters. I was ready to die for my beliefs."
Released in 1989 with nine other fellow Rivonia Trialists, in 1994 he served as an MP for several years until his retirement in 2014. In 1992 he was conferred with Isithwalandwe/Seaparankoe, the highest honour bestowed by the ANC on distinguished members.
He will be remembered as an ethical leader who took an unambiguous stance against corruption during former president Jacob Zuma years. A humble man who never abandoned his township roots, he lived in the same house in Dube.
Residents will remember him as a philanthropist who organised Christmas lunches and donated blankets for the elderly and less privileged. The father of four who lost his wife, June, in 2001, will also be remembered as a keen golfer who enjoyed his smoke way into advanced age.