She had suffered a stroke and a heart attack in August, and was last seen in public on 14 January at a dinner marking the Martin Luther King Jr national holiday, where she received a standing ovation from the 1500 people in the crowd.
King's steely determination, grace and class won her millions of admirers inside and outside the civil rights movement.
John Lewis, a Democratic congressman from Georgia and civil rights leader, said it was "a very sad hour." "Long before she met and married Dr King, she was an activist for peace and civil rights and for civil liberties," he told CNN.
"She became the embodiment, the personification (of the civil rights movement after Dr King's death) ... keeping the mission, the message, the philosophy ... of nonviolence in the forefront."
At the White House, Dan Bartlett, counselor to the president, told Fox television: "President Bush and first lady Laura Bush were always heartened by their meetings with Mrs King. What an inspiration to millions of people. I'm deeply saddened by today's news."
Coretta Scott King played a major back-up role in the civil rights movement until the death of her husband, who was assassinated on a Memphis motel balcony on 4 April, 1968, while supporting a sanitation workers strike.
Coretta Scott and Martin Luther
King married in 1953
Mrs King, who was in Atlanta at the time, learned of her husband's shooting in a telephone call from Rev Jesse Jackson, a call she later wrote, "I seemed subconsciously to have been waiting for all of our lives."
As she recalled in her autobiography My Life With Martin Luther King Jr, she felt she had to step fully into the civil rights movement.
"Because his task was not finished, I felt that I must rededicate myself to the completion of his work," she said.
Determined to make sure Americans did not forget her husband or his dream of a colour-blind society, she created a memorial and a forum in the Martin Luther King Jr Centre for Nonviolent Social Change in Atlanta.
The centre has archives containing more than 2000 King speeches and is built around the King crypt and its eternal flame.
Coretta Scott was born on 27, 1927, near Marion, Alabama. Spending much of her early years on a farm she saw little racial prejudice until she reached high school, when she and her sister were sent into town to board with a family while attending Lincoln High School, one of the black schools in the segregated South.
"It was awful," she said of living in Marion. "Every Saturday we would hear about some black man getting beat up, and nothing was done about it."
Her father had built up a small trucking business but his success began to irritate poor whites in the area, she said, and, after considerable harassment someone burned down the Scott home on Thanksgiving night 1942.
Her sister Edythe won a scholarship to Antioch College in Yellow Springs, Ohio, in 1943, the first black student to attend the school. Two years later, Coretta followed.
But, as the first black at the school to major in elementary education she ran into racial discrimination that limited the classrooms where she could student teach.
King, who was studying for his doctorate in theology at Boston University, had told a mutual friend he was looking for a wife. The friend gave him Coretta Scott's phone number, but when he came calling she was not impressed.
"I saw this green car coming up the street and this short man. He leaned over to open the door, and when I got in the car I saw this very young looking man. I thought, 'Oh my God, I expected to see a man but this is a boy.'"
When he began to speak, however, the young Miss Scott changed her mind.
They were married at her parents' home on 18 June, 1953, and moved to Atlanta, where King was the co-pastor at Ebenezer Baptist Church with his father.