Willie Louis, 76, fled to Chicago and changed his name after testifying at a trial in which an all-white jury acquitted two men who later admitted to Till's murder, the Chicago Tribune reported Wednesday.
Louis, who was known as Willie Reed at the time, saw two white men driving a truck into a barn with two black males in the back on Aug. 28, 1955.
"He heard all this hollering and screaming until there was no more hollering and screaming," his wife Juliet said.
Louis' grandfather told him not to tell anyone but Louis agreed to testify in court and an African-American doctor hid him during the trial.
"He said he couldn't have lived with it; he had to tell them what he saw," Louis' wife said.
When it was over, he fled to Chicago. He died at Advocate Christ Hospital in Oak Lawn, Ill., Thursday. Visitation was scheduled Wednesday evening at New Commandment Church of God in Christ in Chicago.
The reaction to Till's brutal slaying in the racially-segregated town of Money, Miss., and his open casket funeral in Chicago that revealed the disfigured corpse provided impetus to the growing civil rights movement the United States.
"He [Louis] was an unsung civil rights hero," said Mike Small, a friend of Louis and history teacher who has studied the Till case.