In a statement, Lee's family said, "The family of Nelle Harper Lee, of Monroeville, Alabama, announced today, with great sadness, that Lee passed away in her sleep early this morning. Her passing was unexpected. She remained in good basic health until her passing. The family is in mourning and there will be a private funeral service in the upcoming days, as she had requested."
Added nephew Hank Conner in the statement, "This is a sad day for our family. America and the world knew Harper Lee as one of the last century's most beloved authors. We knew her as Nelle Harper Lee, a loving member of our family, a devoted friend to the many good people who touched her life, and a generous soul in our community and our state. We will miss her dearly."
To Kill a Mockingbird is considered one of the great classics of 20th century American literature, and is standard reading in classrooms across the world.
Mockingbird, which was published in 1960, was drawn from elements of Lee's childhood in Monroeville.
In steady prose shaded by memory and lyricism, she describes how an impulsive girl, Scout Finch, her older brother, Jem, their friend Dill and a variety of other townspeople get caught up in the case of Tom Robinson, a black man who's been accused of rape in the Depression-era town of Maycomb, Alabama.
In a statement, her publisher, HarperCollins said: 'The world knows Harper Lee was a brilliant writer but what many don't know is that she was an extraordinary woman of great joyfulness, humility and kindness."
Company's president and publisher, Michael Morrison said: "She lived her life the way she wanted to -- in private -- be surrounded by books and the people who loved her. I will always cherish the time I spent with her."
In 2007, Lee was awarded a Presidential Medal of Freedom from then President Bush in the White House, and she was honoured with a National Medal of Arts in 2010.
After years of being left alone, Lee was catapulted back into the public eye last year with the release of Go Set a Watchman, a "newly discovered" novel she wrote in the 1950s and only the second book from her.
Watchman essentially is an early draft of Mockingbird; most shockingly, it revealed Atticus Finch to be a racist. The controversy over the novel, and worldwide publicity over the reemergence of the elderly writer, made Watchman an instant best seller. It also increased sales for Mockingbird, the perennial classroom classic.