Mars, 84, had been sick for some time, the Neshoba Democrat reported.
She was best known as the author of "Witness in Philadelphia," a book about the 1964 killings of James Chaney, Andrew Goodman and Michael Schwerner. Turner Catledge, who left Neshoba County to become an executive editor at the New York Times, said in his forward to the book that Mars "witnessed with her eyes, her ears and her heart."
Mars was born in 1923 in Philadelphia. Miss., to parents whose families had lived in the area for generations. She graduated from the University of Mississippi in 1944.
While she spent most of her life in Mississippi, Mars worked for an airline in Atlanta for 18 years and as a photographer in New Orleans. In 1957, she bought the Neshoba County Livestock Sale.