Willie Reed died last week at the age of 76.
He was 18 years old when he witnessed the murder of Till in Mississippi.
Reed moved to Chicago after the trial.
He told prosecutors that he saw the two men charged with Till's murder driving a truck with the boy in the back, and then later heard screams coming from a nearby barn.
Despite fearing for his life, Reed came forward and testified at trial.
A memorial service takes place Wednesday night at 6 at a South Side church.