Nkoli was born in Soweto in a seSotho-speaking family. He grew up on a farm in the Free State and his family later moved to Sebokeng. Nkoli became a youth activist against apartheid, with the Congress of Youth African Students and with the United Democratic front.
After coming out in an interview with City Press (1981), in 1983 he joined the mainly white Gay Association of South Africa (GASA), then he formed the Saturday Group, the first black gay group in Africa.
As he spoke at rallies in support of rent-boycotts in the Vaal townships, in 1984 he was arrested and faced the death penalty for treason with twenty-one other political leaders in the Delmas trial. By courageously coming out while a prisoner, he helped change the attitude of the African National Congress to gay rights.
While many gay groups around the world wrote to him and supported his defence, the GASA and white-dominated gay organisations in South Africa refused to support someone charged with political crimes. He was acquitted and released from prison in 1988.
He founded the Gay and Lesbian Organisation of the Witwatersrand (GLOW) in 1988, and was later involved in black gay choirs and sport groups. He traveled widely and was given several human rights awards in Europe and North America. He was a member of ILGA board, representing the African region.
He was one of the first gay activists to meet with President Mandela in 1994. He helped in the campaing to terain the inclusion of protection from discrimination in the Bill of Rights in the 1994 South African constitution, and for the May 1998 repeal of the sodomy law.
After becoming one of the first publicly HIV-positive African gay men, he initiated the Positive African Men group based in central Johannesburg. In the months before his death, supported by his British lover Rodrick Sharp, he was writing his memoirs, and was concerned with anti-homosexual campaigns in neighbouring Zimbabwe, Namibia, Swaziland, Botwana and Zambia. Nkoli died in hospital in Johannesburg.