10 May 1994
After South Africa's first national democratic elections in April 1994, Nelson Mandela was sworn in as South Africa's first democratic President at the Union Buildings in Pretoria on 10 May 1994.. He replaced the outgoing National Party leader F.W. de Klerk as President. Mandela spent the first 18 of his 27 years in jail at the Robben Island Prison. In June 1990 he retired from politics at the age of 80.
References

South Africa info, The world at Mandela’s feet, from Sowetan Newspaper, [online],Available at www.southafrica.info[Accessed: 6 May 2014]