Not OP, but unfortunately the ANC made deals with the neoliberals, which made them not a threat to the US political and financial establishment.

I highly recommend this essay from the late John Pilger (and his interview with Mandela in it). An excerpt:

What was forcing “pragmatism” on De Klerk were the signals from Washington. American companies pumped 40% of the oil that powered apartheid, and supplied the computers that ran the police state, and the trucks and armoured vehicles that attacked the townships. At the UN, America protected South Africa by vetoing hostile Security Council resolutions. And when the regime developed nuclear weapons, Washington winked.

At 4.16 pm on 11 February 1990, Mandela walked free. He wanted an extra week in prison to prepare himself, but De Klerk said no. He was bundled out. When he stepped out onto the balcony of Cape Town City Hall, he reached for his spectacles and realised he had left them in prison. Wearing his wife’s glasses, and with Cyril Ramaphosa supporting him, he spoke to millions in South Africa and around the world. “Now is the time to intensify the struggle,” he said, warning the regime that if its orchestrated violence continued, “the people will not hesitate to fight back.” It was a proud and angry statement and perhaps the most militant speech Mandela ever made.

The next day he appeared to correct himself. Reassuring the white establishment that he was “not a communist” and that majority rule would not result in “the domination of whites by blacks”, he repeated his earlier description of De Klerk as “a man of integrity”. This upset many in the resistance, and when word spread that he and Mbeki had been secretly negotiating for more than two years, there was widespread disappointment and dismay. This turned to anger when it was revealed that Mandela had written to P. W. Botha offering special constitutional protection for whites.

“Do you recognise that many people saw this as betrayal?” I asked Thabo Mbeki during an interview. He replied: “Had we not made the historic compromise, there would have been bloodbath and a great suffering across the land.”

While it is true there was no civil war, the political decisions made by Mandela, Mbeki and their fellow “moderates”, have allowed the continuation of suffering by exclusion: apartheid by other means. Over the course of three years, half a dozen critical decisions were made by a small group around Mbeki (who was advising Mandela), Finance Minister Trevor Manuel, and Trade Minister Alec Erwin. These were, in 1992, to drop nationalisation, to endorse the apartheid regime’s agreement to join the General Agreement on Tariffs and Trade (GATT, the forerunner of the WTO), which effectively surrendered economic independence and, in the same year, to repay the $25bn of apartheid-era debt, grant the Reserve Bank formal independence, and accept loans from the IMF, and in 1995, to abolish exchange controls which allowed the wealthy whites to take their capital overseas.

Anti apartheid sanctions were passed pretty late in the USA, around 1986. Nelson Mandela was elected in 1994. Reagan tried to veto the anti apartheid bill, and got then South African foreign minister Pik Botha to call US politicians on the fence about the veto. This backfired and Reagan's veto was overruled, and the anti apartheid legislation passed. However, South Africa was subject to a voluntary arms embargo by the United Nations security council since 1963, an embargo the US voted in favour of. Only the UK and France abstained. This is why South Africa had such outdated military equipment by the end of apartheid, their newest fighter aircraft at the time were both of UK and French design, in the Buccaneer and Mirage respectively. This is probably a reason for the abstention. South Africa later attempted a domestic modernisation programme on some of it's weapons using Israeli technology. (The Atlas Cheetah is the best example of this). However the US did covertly support apartheid for many years.