Mr Obama, his wife and two daughters viewed the lime quarry where Mandela toiled during captivity. It's also where the 94-year-old anti-apartheid leader developed the lung problems ailing him today.
The Obamas' tour guide is 83-year-old politician Ahmed Kathrada, who was also held captive at the prison. He told the Obamas the prisoners were forced to work every day.
Mr Obama has been paying tribute to Mr Mandela throughout his weeklong trip to three African countries. Obama will give a major speech later today at the University of Cape Town. The White House says the speech will be infused with memories of Mandela.