During the last six weeks Machel has kept a near round-the-clock vigil at her husband's hospital bedside, leaving to attend Mandela's charity events or visit their Johannesburg home.
But then the Mozambican born Machel, 27 years younger, has already been by Mandela's side for 15 years of marriage.
"We make sure we spend time with each other because we were so lonely before. You only live once," she remarked before the ravages of time tightened their grip on her husband.
Machel married Mandela in 1998, 12 years after her first husband, Mozambique's former president Samora Machel, died in a mysterious plane crash.
At that time Mandela had been divorced from his second wife Winnie for two years, but estranged by time and prison for three decades.
"When I am alone, I am very weak," South Africa's first black president said when discussing Machel in 2007.
On the cusp of their anniversary, Mandela's close friend and lawyer George Bizos described them as "a loving couple."
"They are very happy together," he told AFP, adding Machel "is dedicated to the well-being of her husband Nelson. It's commonly known that she spends a long time at his bedside."
While South Africa's revered former statesman battles to recover, Machel has been notably absent from the unseemly squabbles rocking the Mandela family.
As she prepares for the dreaded, yet inevitable, she has largely stayed out of the public eye.
But at one charity event she gave a rare, but desperately awaited, first-hand account of Mandela's condition.
He was, she said, at times uncomfortable, he had seldom been in pain.
Last week she sounded an even more upbeat tone, saying she was now less anxious about his health.
The two met in the early 1990s and fast became friends.
As Mandela's second marriage withered their relationship grew.
Eventually, after being spotted at several events holding hands and even stealing a kiss at Robert Mugabe's wedding, the presidency declared Machel was officially Mandela's companion.
Despite her remarkable intellect and impressive history of work promoting literacy, the bespectacled Machel attracted her share of scepticism from South Africans.
This, after all, was a nation used to the flamboyant Winnie Madikizela-Mandela, who even joined in the fray mocking her as Mandela's "concubine."
At their wedding on July 18, 1998, archbishop emeritus Desmond Tutu said Machel had "made a decent man out of" Mandela.
'Tremendous debt of gratitude'
He would later say that "South Africans owe Graca Machel a tremendous debt of gratitude for the joy she has brought to Nelson Mandela since their marriage."
As Mandela grew frailer Machel began to shield him from the ugly family feuds and unwanted attention.
"He is an avid TV watcher and newspaper and magazine reader, and she carefully protects him from any publicity which would disturb him," said Bizos.
"She is very caring, she doesn't want him to be exposed to any unpleasant reports."
An independent woman, who wears a broad smile and sports a short afro, Machel chose to keep her former husband's surname after she married Mandela.
She has refused to paint Mandela as a "saint" saying he is "just a human being who is simple and gentle."
She is in the book of Guinness world records as the first woman to be first lady of two different countries.
Born in Mozambique's Gaza province to a humble rural family, she is the last of six children.
She never saw her Methodist lay priest father, who was a migrant worker and died three weeks before she was born.
She won a Methodist church scholarship to study at Lisbon University where she cut her teeth in political activism against Portuguese colonialism.
Machel joined the liberation war movement Frelimo and received military training in Tanzania in the 1970s.
She later became education minister in independent Mozambique.
She has now become a global humanitarian and political heavyweight and an internationally sought after speaker.
In 2010 Time magazine named her among the world's 100 most influential people.
A UN expert on children in armed conflict, she is also leading a global campaign against child brides.
She is fluent in English, French, Italian, Portuguese, Spanish and her native Tsonga.