(ABC News)

Former President Bill Clinton counts Nelson Mandela as a "true friend" during the six years of their concurrent presidencies and after.

Reflecting on Mandela's life and presidency, Clinton told ABC News that Mandela disciplined himself to keep his anger toward the people who jailed him for 27 years private.

"He realized that if he brought his personal feelings into it, he couldn't do what was right by the people that he had gone to prison for 27 years for," Clinton told ABC News' George Stephanopoulos in an interview airing tonight on "World News with Diane Sawyer." "He realized it wouldn't work."

"And he also realized that he personally and his country could never be free without forgiveness," Clinton added.

That doesn't mean he had no lingering feelings toward his jailers and the regime that enforced the brutal system of apartheid.

Clinton said that Mandela would on occasion reveal "flashes of anger" about the hard labor and his mistreatment in prison to his friends, but rarely would it come to the surface.

"He once told me that he lived on hatred when he went into prison because he was young and he was being abused and he was out there cracking those rocks all day and he said after about 11 years he realized that they'd already taken about everything they could take from him except his mind and his heart," Clinton said.

"He said I realized that those are things you have to give away and I decided not to give them away."

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Mandela is known for having invited his jailers to his inauguration, a symbolic gesture that helped start the process of healing in South Africa. But he also urged reconciliation in the way he governed.

"[A]s a politician and a president, more important was the fact that he put the leaders of the parties who had imprisoned him in his cabinet," Clinton said. "He left the structure of the government intact as long as people were willing to work with him to achieve the objectives of the country."

Since leaving office, Clinton has traveled to South Africa nearly every year, often working with Mandela on AIDS programs, and spending time with Mandela around his birthday.

The two presidents shared a close bond during their presidencies, Clinton revealed. He would talk to Mandela late at night-when it was a reasonable hour in South Africa-to confer on policy and to draw personally on Mandela's wisdom.

"He was always trying to help me deal with the challenges and always reminding me that it wasn't just all those years he spent in prison where what went on in your mind and heart was the most important thing," Clinton said. "Sort of a precondition for being able to do any good for anybody else."