I grew up and spent a major part of my adult life in the U.S., where the legacy of slavery and apartheid was (and remains) a constant presence. Until my twenties I lived in Cudahy, Wisconsin, a working- and lower-middle class suburb of Milwaukee that, according to reliable reports, had the children of exactly one black family in its large public high school, which drew from a population of 20,000. (I did not attend the high school, and never saw those children with my own eyes.) Young children who had rarely, if ever, encountered a black person would call each other "nigger" as a joking insult. (I did not join in the racist language. My parents would not have tolerated it.) Blacks did not stay away from Cudahy because it would have been dangerous or uncomfortable for them to live there; they stayed away because it was unthinkable for them to live there. Even in my, relatively speaking, enlightened family, whose breadwinner was in the residential real estate business, no one found the absence of blacks in Cudahy at all peculiar.
We do not have to ignore the remaining serious problems of race in the U.S. – highlighted by current brazen methods of suppressing minority voting in many states – to recognize that tremendous progress has been made.
Nelson Mandela's impact was immeasurable both in the sense that we cannot judge with any precision how much the American Civil Rights movement gained strength from the protests against apartheid in South Africa, and in the sense that this impact was nevertheless undoubtedly very great. If anyone initially did not "get" racial injustice with respect to the 20th Century U.S., they usually got it about South Africa. If anyone initially did not get the heroism and nobility of American civil rights leaders, they usually got it about Nelson Mandela. He helped everyone see.