19 March 1996
After 38 years of marriage, Judge Eloff granted Nelson and Winnie Mandela a divorce.  Irreconcilable differences were cited as the reason for the parting. The Mandelas were married in 1958 and only spent a couple of years together before Nelson Mandela was sentenced to life in prison as one of the Rivonia Trialists in 1963. It is believed that the lengthy separation that the couple experienced while Nelson Mandela was incarcerated contributed greatly to the eventual split. Two daughters, Zenani and Zindzi were born from their marriage. 
References

Daily, S. (1996), South African Judge Gives Nelson Mandela a Divorce, from The New York Times, 20 March [online], Available at www.nytimes.com [Accessed: 08 March 2010]|Block, R. (1996), Mandela: Winnie made me the loneliest man, from The Independent Online, 19 March, [online], Available at independent.co.uk [Accessed: 08 March 2010]