Sontag, who had been suffering from cancer for some time, died on Tuesday at a New York cancer hospital. She was 71.
She was known for interests that ranged from French existentialist writers to ballet, photography and politics, and once said a writer should be "someone who is interested in everything".
Sontag called herself a "besotted aesthete", an "obsessed moralist" and a "zealot of seriousness".
The author of 17 books, including the best-selling historical novel, The Volcano Lover, she won the US National Book Award in 2000 for the historical novel In America.
But her greatest literary and cultural impact was as an essayist.
Her 1964 study of homosexual aesthetics called Notes on Camp established her as a major new writer.
The essay introduced the "so bad it's good" attitude towards popular culture, applying it to everything from Swan Lake to feather boas.
Her work has been translated into more than 30 languages.
Sontag was among the first to raise a dissenting voice after the September 11 attacks, in a controversial New Yorker magazine essay.
"She was brilliant and put her brilliance to work on behalf of human rights and creativity for everybody else"
Victor Navasky, publisher of weekly magazine the Nation
She ignited a firestorm of criticism when she declared that the attacks were not a "cowardly attack" on civilisation but "an act undertaken as a consequence of specific American alliances and actions".
Sontag has since been an outspoken critic of US President
George Bush over his response to the attacks and particularly the US war in Iraq.
In May this year she wrote an essay in The New York Times
about the prisoner abuse scandal at Abu Ghraib jail in Baghdad, arguing that the shocking photographs of the abuse would very likely become the defining images of the war.
Novelist EL Doctorow described her as "quite fearless".
"She was engaged as a writer. I remember she went to Sarajevo to do theatre while the fighting was going on. She just marched right on in there," he said.
A long-time opponent of war and a human rights activist, Sontag made several visits to Sarajevo and staged Beckett's Waiting for Godot there under siege in 1993.
Brilliant for others
Born in New York in 1933, Sontag grew up in Arizona and Los
Angeles before going to the University of Chicago, and later
Harvard and Oxford University in England.
She was married at the age of 17 to Philip Rieff, an academic in Chicago, with whom she had a son in 1952.
"She was brilliant and put her brilliance to work on behalf of human rights and creativity for everybody else," said Victor
Navasky, publisher of weekly magazine The Nation.
From 1987 to 1989 she was president of the American Centre
of PEN, an international writers' organisation dedicated to
freedom of expression, where she led a number of campaigns on behalf of persecuted and imprisoned writers.