Nelson Mandela’s arrest in 1962 came as a result of a tip-off from an agent of the US Central Intelligence Agency (CIA), a report says.
The revelations, made in the Sunday Times newspaper, are based on an interview with ex-CIA agent Donald Rickard shortly before he died.
Mr Mandela served 27 years in jail for resisting white minority rule before being released in 1990.
He was subsequently elected as South Africa’s first black president.
The interview appears to confirm long-held suspicions that Mr Mandela was being trailed by the CIA, says the BBC’s Karen Allen in Johannesburg.
It is expected to put pressure on the CIA to release documents about its involvement in Mr Mandela’s arrest and support for the apartheid government, although it has resisted previous calls for their disclosure.
Mr Rickard, who died earlier this year, was never formally associated with the CIA but worked as a diplomat in South Africa before retiring in the late 70s.
The interview was conducted by British film director John Irvin, who has made a film about Mr Mandela’s brief career as an armed rebel, the Sunday Times said.
The future president led the armed resistance movement of the banned African National Congress (ANC), and was one of the most wanted men in South Africa at the time of his arrest.
Mr Mandela was considered a dangerous communist and a threat to the West, our correspondent says, although he always denied ever being a member of the communist party.
His ability to evade the security services had earned him the nickname “the black Pimpernel”.
He was posing as a chauffeur when his car was stopped at a roadblock by the police in the eastern city of Durban in 1962 and he was detained.
Mr Mandela, president of South Africa from 1994 to 1999, was on a US terror watch list until 2008.
Before that, along with other former ANC leaders, he was only able to visit the US with special permission from the secretary of state, because the ANC had been designated a terrorist organisation by the former apartheid government.
The bill scrapping the designation was introduced by Howard Berman, chairman of the House Committee on Foreign Affairs, who promised to “wipe away” the “indignity”.
President Ronald Reagan had originally placed the ANC on the list in the 1980s.