A police officer told him to drop the knife. He didn't listen, and the officer fired on him out of fear for his life, according to a police union spokesman.
McDonald, 17, died. He was shot 16 times.
Now, more than a year later, the public will be able to see what happened.
On Thursday, a judge in Chicago ordered the city to release no later than November 25 the police dashcam video that shows the shooting. For months, the city has fought attempts to have the video released to the public, saying it could jeopardize any ongoing investigation.
However, in issuing his 18-page decision, Cook County Judge Franklin Valderrama said the city failed to prove that claim. Valderrama also denied a request by the city's attorneys to issue a stay in his decision while they appeal to the Illinois Appellate Court.
The officer who shot McDonald, Jason Van Dyke, has not faced charges and still works for the police department in a "limited duty position."
'Graphic, disturbing and difficult to watch'
Almost everyone who has seen the video says the images are graphic and show McDonald being repeatedly shot while lying on the ground. The video reportedly shows McDonald's body palpitating quickly several times, consistent with shots striking. Even defense attorneys for the officer involved admit the video is not easy to watch.
"The video is graphic, disturbing and difficult to watch, as any video of a man being shot to death would be," Daniel Herbert, the officer's attorney, told the Chicago Tribune.
Herbert told CNN on Friday that before his death, McDonald had "punctured a tire on a police car."
"At the point which my client confronted Mr. McDonald, my client was aware of the fact that the individual (McDonald) had not complied with numerous police orders to drop the knife," Herbert said.
Herbert defended Van Dyke's actions, saying the officer "believed he was in fear for an attack and for the safety of anyone else on the scene."
Mayor Rahm Emanuel's office issued a statement Thursday afternoon criticizing the officer.
"Police officers are entrusted to uphold the law, and to provide safety to our residents. In this case unfortunately, it appears an officer violated that trust at every level."
After spending months trying to keep the video from going public, the city abruptly announced it would not appeal the judge's decision. In agreeing to release the video by November 25, the statement expressed hope that prosecutors would have time to finish their investigation so "Chicago can begin to heal."
Thursday's decision was the result of a Freedom of Information Act request filed earlier this year by freelance journalist Brandon Smith. Smith's attorneys applauded the decision.
"There's a tremendous public interest in this," an attorney for Smith said. "The public should not be at the mercy of when the police department dictates the video should be released."
Investigating the case
Authorities said in April that a joint investigation into McDonald's death would be conducted by federal and state authorities, spearheaded by the Chicago branch of the FBI.
CNN reached out to the U.S. attorney's office investigating the case, who said they had nothing new to report since the April announcement.
The Cook County State Attorney's office would not comment to CNN.
Herbert, Van Dyke's attorney, would not speak with CNN about which authorities are investigating the shooting.
"He's scared to death, but more than himself he's scared for his wife, his two kids," Herbert said of Van Dyke. "He knows in his heart of hearts that his actions were appropriate."
The city has already reached a settlement with McDonald's family, agreeing in April to pay $5 million, though the family had not filed a lawsuit.
Preparations for video's release
Emanuel's office has not released details on when or how it plans to release the video.
But some in the city are bracing for protests and unrest.
John Kass, a columnist for the Chicago Tribune, said the video
"could tear Chicago apart."
Dean Angelo, the president of the Chicago Fraternal Order of Police, told CNN affiliate WBBM
he is concerned protesters from outside the city may converge to "disrupt and cause problems."
"This is my city. We all live here," Angelo said. "I'm concerned about the safety of my officers. I'm concerned about the safety of the civilian population. ... I think local people don't want to see their neighborhoods damaged."