Andrew Mokete Mlangeni was born on 6 June 1925 at Prospect township in Soweto. He was the ninth child in a family of fourteen and his father passed away in 1936 shortly after his mother gave birth to a set of twin daughters. Although Mlangeni started schooling at the age of 10, financial problems drove him to seek work at the age of 12 to assist his mother in maintaining the family. After school he worked as a caddy at the Johannesburg Golf Course. In 1942 his elder brother who was staying in Pimville Soweto, assisted in paying his school fees. At that time he was a student at St. Peters Secondary School, where he obtained his Junior Certificate in 1946.
Faced by poverty and financial problems, Andrew was unable to continue with his studies. After 1946 he worked in several industries and factories where he experienced worker exploitation. As a result, when working as a bus driver for Putco he was active in the strike for better working conditions and a living wage. In 1951 he joined the African National Congress Youth League (ANCYL) and later in 1954 he joined the ANC. During the Congress of the People he was a branch delegate at Kliptown. From 1958 to 1960 he was an ANC stalwart and in 1961 he was among the first to be sent for military training outside the country. On his return in 1963 he was arrested after state witnesses told the court that he was one of the people responsible for recruiting and training an armed force. He was found guilty and sentenced to life imprisonment on Robben Island. Mlangeni was however released when negotiations between the ANC and the government began.
Before the court passed judgment on him, Mlangeni told the court that:
"Though leaders of many countries throughout the world have tried to persuade the Government to abandon its apartheid policy, and although resolutions have been passed in the United Nations against South Africa, this has met with no result. All that the Government has done is to reply to the people's demands by putting their political leaders in gaol, and breaking up families"
At time of his sentence Mlangeni was married to Johanna Junny with whom he had ten children. His wife died in 2001.