He has changed hers too. "I am very happy," she said this week in London, beaming when the conversation moved from the unhappy subject of Third World debt, which is this weekend on the agenda of the world leaders gathered in Birmingham for the G8 summit. She is an elegant animated woman who laughs a lot, when the moment is apt.
Her friendship with Mr Mandela - whom she calls by the affectionate nickname Mandiba - began in 1986 after the imprisoned leader wrote from Robben Island expressing his condolences over her husband's death. She had been left with a son and a daughter, aged seven and 10, and five step-children.
When they met for the first time in 1990 soon after his release both were aware on an instant rapport which began to grow into a friendship when they met again in 1992 when Mrs Machel received an honorary degree from a South African university.
Then in 1993 the African National Congress president, Oliver Tambo, who was godfather to her children, died and Mr Mandela took over as the role and the relationship between the couple grew. After it became clear that Mr Mandela's marriage to Winnie was beyond repair, he began to go Mozambique for the weekend "to get away". The couple realised they had fallen in love.
He launched a fresh investigation into her husband's death. An official South African report at the time blamed pilot error, but there was talk of bullet-holes in the fuselage. How was it going? I asked her. "It is a very complicated issue," she said. "We're dealing with it, quietly."
But what began the progress which has brought her to love and fulfilment was something different. It was a tiny charity called the Foundation for Community Development. It is what, in the two weeks every month that she is apart from Mr Mandela, consumes her time.
"It is very small. We give grants and loans to small groups of farmers and women to help them generate income for themselves," she said. "We help in kind too, giving goats and cattle from which they breed until they can afford to return the number they were given. We help train and equip women in sewing. And we help build up the skills of indigenous NGOs."
At first sight it may seem an odd move for the woman who was once a guerrilla in the fight to liberate the country from Portuguese rule and then became the only female in the Mozambican cabinet - she was education minister for many years, even carrying on for three years after her husband's death. In 1989, she persuaded the new president to accept her resignation and withdrew from public life.
"I had to rethink, to gather all the scattered pieces of myself. I had to acknowledge that somehow I had been handicapped, that part of myself was no longer there. As minister of education I dealt with policy formation and macro-economics. And, yes, at the end of the year you know how many people graduate, but you don't touch any one in your everyday life. But here the people I deal with each have a face. It is a two-way process; I receive from them the lessons that they have to teach. It is more human and much more fulfilling. Through this small charity, with only a $1.5m turnover, I found a meaningful way of continuing to be alive."
Her combined understanding of macro-economics and of its impact at the lowliest level gives her a unique position from which to speak out this weekend in Birmingham at the G8. "In recent years the macro-economic indicators in Mozambique have been improving, with annual growth of between 6 and 8 per cent and inflation down to 4 per cent. But that doesn't mean the lives of ordinary people are improving. Quite the opposite. Living standards are worsening."
Think about Britain, she said, and imagine that "35 million people here have no access to clean water - and 20 million women cannot now read or write. That's what it is like for us. Yet we have to pay $100m a year in debt repayments - which is more than we spend on health and education combined. The IMF and World Bank don't have the courage to acknowledge the ill-effects of the remedy they are imposing. But on the ground you'd have to be blind not to see it. Many of these people aren't going to survive till we reach the long run." According to the World Bank's Human Development Report an extra 21 million people will die in the Third World between now and 2000 if debt relief is not secured. How can we postpone our right to live? It's not negotiable. The high rate of malnutrition affects the brain - these children may live physically but they will be handicapped intellectually. Such things are criminal. They are silent ways of killing."
Finding the time to carry on her work - and she has recently completed a report for the UN on the fate of children caught up in war - is not easy when she is now involved in a relationship with Mr Mandela which involves international commuting and daily telephone calls during their frequent separations.
"It's difficult," she said. "But it's only a 45-minute flight from Maputo to Johannesburg. The real difficulty is having two families, but I love my children and I love him too. If you love somebody you find a way of doing it."
As to reports from Mozambique that she would be standing for president in 1999, they were "nonsense". It is the year in which Mr Mandela will retire and from which point she hopes to help him experience the normal family life he has never had - and find the time to enjoy with him the things he loves doing for which his office never gives him time.
And would they be getting married? "I don't want to talk about it," she said with a squeal of laughter. Was she still talking to Mr Mandela's fellow Nobel Peace Prize winner, Archbishop Desmond Tutu, who had publicly complained about their "intimate relationship outside marriage"?
"He's a good friend to Mandiba and he's a good friend to me. He was, and he still is. Of course, we don't ignore what he says, but I don't think we're a bad example to the youth of South Africa. I don't think they see us that way. I don't think it affects the way people in Mozambique think about me and it certainly doesn't lessen Mandiba's authority."