CAPE TOWN, South Africa — As Ahmed Kathrada led President Obama and his family recently through the prison on Robben Island where Mr. Kathrada had spent much of his life, he explained how the rules of apartheid had granted him, because of his Indian ancestry, long pants and socks. One of his fellow inmates, Nelson Mandela, as a black man, received short pants and no socks.

Mr. Kathrada, 83, also showed the Obamas the sign listing the different amounts of sugar, coffee, soup and other foods that South Africa’s prison system had apportioned to blacks; mixed-race inmates, who were known as coloreds; Indians; and whites.

“In everything there was apartheid,” he said in an interview on Thursday in his small apartment here in the shadow of Table Mountain....