Nelson Mandela’s official title as South Africa’s head of state from 1994 to 1999 was President.
He took office on 10 May 1994 after the country’s first democratic election. The 1993 Interim Constitution created the office of State President, but the 1996 Constitution established the modern title President of the Republic of South Africa, which Mandela continued to hold.
Mandela served as both head of state and head of government. His deputy presidents were F. W. de Klerk and Thabo Mbeki under the Government of National Unity. He left the presidency on 16 June 1999, when Mbeki succeeded him.
“Prime Minister” is a common but incorrect answer. South Africa had used prime ministers in an earlier constitutional era, but Mandela’s office was the presidency during the democratic transition.