Mars was one of the few whites in Neshoba County, Mississippi who supported the Civil Rights Movement.
Edgar Ray Killen and others faced trial for violating the civil rights of the men in 1967 but the jury was deadlocked.
Mars' activities led to the Ku Klux Klan burning down her barn in an effort to intimidate her.
In 1977, she published Witness in Philadelphia which told of her activities in the civil rights movement and the murder of the three activists.
This book helped to keep interest alive in the murders with the success of Mississippi Burning making many more people aware of the incident.