30 January 1990
Clayton Sithole was the last political activist to die in detention. He died in John Vorster Square police station just 12 days before the release of former president Nelson Mandela. At the time of his arrest, Sithole was in a relationship with Zindzi Mandela, the daughter of Nelson and Winnie Mandela, and had fathered her son, who was less than a year old at the time of his death.

He was found hanging by a belt and shoelaces from a water pipe in the shower. His death was ruled a suicide. Sithole was just one of the many detainees to have died in John Vorster Square police station, in Johannesburg, Transvaal (now Gauteng). This police station gained a reputation as many detainees seemed to perish inside the building. In none of the inquests held regarding his, and other deaths, did the blame fall on the police, despite many suspicions that they were responsible. 
References

Sunday Times Heritage Project, Going to Goch Street, from The Sunday Times, [online], Available at heritage.thetimes.co.za [Accessed: 9 January 2014]|Burns, J.F., (1990), South Africa Appoints Judicial Inquiry in Death of Black Close to Mandelas, from The New York Times, 01 February, [online], Available at www.nytimes.com [Accessed: 9 January 2014]| South African Institute of Race Relations. (1990). Race Relations Survey 1989/90, Johannesburg: South African Institute of Race Relations, p. 177