In this Oct. 24, 1955, file photo, officers stand by as black religious leaders from Chicago demonstrate outside the White House in Washington against the murder of 14-year-old Emmett Louis Till. (Associated Press)

Willie Reed did not know Emmett Till, the young black man whose murder in the Mississippi Delta became one of the most infamous lynchings in the history of the Jim Crow South. Mr. Reed saw him only once — on Aug. 28, 1955, during the last hours of Till’s life — in the back of a green and white Chevrolet pickup truck.

Mr. Reed, an African American sharecropper, risked his life at 18 to appear as a surprise witness in the prosecution of the white men accused of the crime. He became the momentary hero of the Till trial, an event that helped spur the civil rights movement but left a moral stain on the American legal system.

Mr. Reed died July 18 at a hospital in Oak Lawn, Ill. He was 76, and he had lived in Chicago under a different name — first in secrecy and later in relative obscurity — since fleeing Mississippi for his safety nearly 60 years ago. For decades, he had worked as a hospital orderly.

Till, who would have turned 72 on Thursday, was, in 1955, a Chicago teenager unacquainted with the strain of racism prevalent in the South. Accused of whistling at or otherwise affronting a white woman, he was abducted from his relatives’ home near the hamlet called Money, then beaten and executed.

His body, tethered with barbed wire to a cotton gin fan, surfaced in the Tallahatchie River three days later. His assailants had crushed part of his head and gouged out one of his eyes. He was missing an ear.

Willie Reed, a key witness in the Emmett Till murder trial, died at 76. (United Press Telephoto)

Mr. Reed knew speaking out against the defendants in the case would make him, too, a target for lynching. But he “couldn’t have walked away,” he said years later. “Emmett was 14,” Mr. Reed told the CBS News show “60 Minutes,” “and they killed him. I mean, that’s not right. . . . I knew that I couldn’t say no.”

“He was a brave kid to do what he did,” said Moses J. Newson, a reporter for an African American publication who covered the Till case. “Blacks weren’t expected to do a lot of testifying against white people in court in Mississippi.”

By the time the trial opened in September 1955, images of Till’s disfigured corpse had circulated throughout the nation, horrifying Americans of all races and helping to galvanize the building movement for civil rights. Tens of thousands of mourners paid their respects at his open coffin in Chicago.

“There was no way I could describe what was in that box,” his mother, Mamie Till Mobley, later told an interviewer. “No way. And I just wanted the world to see.”

Back in Mississippi, the white law enforcement establishment failed to mount a vigorous case against the defendants — Roy Bryant, the husband of the woman Till had supposedly offended, and J.W. Milam, an acquaintance of Mr. Reed’s.

With the trial underway, Medgar Evers and other civil rights activists organized an independent search for witnesses who could testify to the defendants’ guilt. Aware of the perils of their mission, the activists disguised themselves as plantation workers when they approached Mr. Reed and persuaded him to appear as a witness for the prosecution.

“You could tell that he was mighty afraid,” Simeon Wright, a cousin of Till’s who shared a bed with him the night he was abducted, told The Washington Post this week. “For him to testify against these men, that was instant death.”

According to news accounts of the trial, Mr. Reed spoke so softly that he could hardly be heard from the witness stand as he recounted what he had seen.

In the early hours of Aug. 28, 1955, a Sunday, Mr. Reed was walking to a store when he saw a Chevy pickup carrying several men, black and white, and a youth he would recognize in newspaper photographs as Till. Mr. Reed later saw the truck parked on a nearby property that belonged to a relative of Milam’s.

“I come on by the barn,” Mr. Reed testified. “I heard somebody hollering, and I heard some licks like somebody was whipping somebody.”

“And what about the licks?” a prosecutor inquired. “Was it just one lick you heard, or was it two, or were there several licks?”

“There was a whole lot of them,” Mr. Reed replied.

Mr. Reed testified that he saw Milam emerge from the barn, a pistol on his belt, to take a drink from the well. At one point, the prosecutor asked Mr. Reed if he saw Milam in the courtroom. Mr. Reed replied that he was “sitting right over there” — and pointed at the bald-headed defendant.

To give such testimony, the prosecutor told the jury, “Willie Reed has more nerve than I have.” Civil rights activists arranged for Mr. Reed to be spirited out of town and taken by train to the North to better secure his safety. He remained under police protection for several months, his wife said, and was hospitalized for a nervous breakdown.

Despite the overwhelming incriminating evidence — including testimony by Till’s great-uncle — the all-white jury acquitted the defendants after little more than an hour of deliberation. One of the jurors said they would have rendered a verdict more speedily if they had not “stopped to drink pop.”

Weeks later, despite the danger and over his mother’s vigorous objections, Mr. Reed returned to Mississippi to testify against the same defendants on separate kidnaping charges. The grand jury declined to return an indictment. The next year, knowing that they were protected by laws against double jeopardy, the defendants confessed to the Till killing in a paid interview with Look magazine.

Willie D. Louis — he had no middle name — was born July 14, 1937, in Greenwood, Miss. After his mother moved to Chicago to seek employment, he was raised by his maternal grandparents and used their surname, Reed.

As quickly as he was pushed into the national spotlight, Mr. Reed left it. To protect himself after the Till trial, he reverted to the birth name he had never used. He worked as a surgical orderly in Chicago area hospitals until 2006.

His wife of 37 years, Juliet Mendenhall Louis, confirmed his death and said the cause was gastrointestinal bleeding. She said that she was married to him for seven or eight years before she learned of his role in the Till case and that his memories burdened him until the end of his life. Sometimes, she said, he would wake up from his sleep “moaning and turning.”

Besides his wife, of Chicago, survivors include a stepson he raised as his child, Sollie McKinnon of Ashtabula, Ohio; seven grandchildren; and 21 great-grandchildren.

Beginning about a decade ago — by which time Milam and Bryant had died — journalists and documentary filmmakers including Stanley Nelson and Keith Beauchamp drew new attention to the Till case. In 2007, after a years-long federal investigation that included the exhumation and identification of Till’s body, a Mississippi grand jury found insufficient evidence to bring new charges.

Wheeler Parker, another Till relative who was sleeping in the house the night of the abduction, said he assumed that Mr. Reed had been lynched after his testimony in the 1955 kidnapping proceedings. Decades later, they reconnected after the resurgence in interest in the Till case, and the two men discovered that they lived just miles apart in Illinois.

“You have to live those times to understand what it was like, the pure terror,” Parker told The Post in an interview . “His stepping forward, his testifying, it was just a very courageous act on his part. It’s beyond words for me to explain.”

GALLERY: Notable deaths of 2013