"Her daughter went in to wake her up and she was not able to and so she quietly slipped away. Her spirit will remain with us just as her husband's has," Andrew Young, a family friend and former civil rights activist said.
Ms Scott King died at 0700 GMT of a heart attack at a holistic clinic in Rosarito, Mexico, said institution director Humberto Simanti.
She had also been fighting ovarian cancer and last year suffered a heart attack and stroke.
Her body was being taken to Atlanta, Georgia for burial.
Revolutionary life
Born April 27, 1927 in segregated Alabama, Ms Scott King grew up on her parents' farm.
Her father, Obediah Scott, was the first black man in the area to own a truck and then launched a truck-farming business, which drew opposition from white neighbours.
Her mother, Bernice, frustrated that buses took only white children to school, rented a bus to transport black children to classes.
In 1945, after graduating from high school, Ms Scott King followed her older sister to College, where she earned a degree in music and elementary education.
Her sister was the college's first full-time black student.
Ms Scott King went on to study at the New England Conservatory of Music in 1951, where she met her future husband.
The promising singer married Martin Luther King Jr in 1953 immediately supporting his equal rights campaign, often speaking in the Nobel Peace prize laureate's place when he was unable to attend an engagement.
She was with him in 1963 when he delivered his famous "I Have a Dream" speech at the Lincoln Memorial in Washington.
Four days after his assassination in 1968, Ms Scott King led an equal rights march by 50,000 people through the streets of the Tennessee city.
In the following years, while raising four children, she campaigned fiercely to keep alive her husband's message of non-violent change.
Tributes
Reverend Al Sharpton, a prominent civil rights activist and family friend, remembered Ms Scott King as a "compassionate, caring, yet firm matriarch of the movement for justice."
Local politicians and scores of people in Atlanta headed to the King Centre, created by Ms Scott King in memory of her husband, laying wreaths in her memory.
President George W. Bush led the political tributes saying in a statement, "Mrs King was a remarkable and courageous woman, and a great civil rights leader."
"Mrs King's lasting contributions to freedom and equality have made America a better and more compassionate nation.”
US Secretary of State Condoleezza Rice, the highest ranking
African-American in the US government, said the world had "lost a champion of human rights."
The US Congress also lowered its flags in her honour in Washington.
A tribute also came from South Africa, where Ms Scott King travelled in 1994 to see Nelson Mandela become the country's first democratically-elected president.
"In our own struggle against apartheid in South Africa, Coretta and Dr Martin Luther King were at all times a towering presence, who, through their experiences, provided guidance, inspiration and, indeed, helped us to maintain the unshakable belief that we, too, would overcome," said South African President Thabo Mbeki.