The Canadian-born reporter was just a few months into a stint covering the White House for NBC in November 1963, and the assignment to accompany U.S. President John F. Kennedy to Texas had been passed up by a more senior colleague who opted to cover a convention in Hawaii instead.
When the events of that trip turned deadly, MacNeil was on hand to hear witness accounts and keep the country briefed on medical updates. He may even have exchanged a few words with the president's assassin.
But the newsman, who became a household name by cohosting the popular MacNeil/Lehrer NewsHour on the U.S. Public Broadcasting Service, said he pulled off one of his toughest journalistic assignments by breaking the rules.
"I was very new to covering the president, and I suppose the iron-clad rule is that if you're covering the president, you stay as close to the president as you can," MacNeil said in a telephone interview from his home in New York City.
"Whereas I had been a foreign correspondent for NBC for several years before that, and in television news, like any kind of news, if something happens you run towards it. The closer you are, the better your television."
That maxim served him well from the moment gunfire echoed through Dealey Plaza. MacNeil said he could not see the action from his position in the motorcade trailing the president, despite being only seven vehicles away at the time.
He ordered the press bus driver to stop the vehicle and let him out, making him the only reporter on board to race towards the action. MacNeil zeroed in on police who had rushed to the scene and followed them over landmarks that would become synonymous with the investigation into Kennedy's death.
He trailed officers over the grassy knoll where some some theories suggest a second assassin may have fired shots that missed their mark.
He followed police past cowering witnesses shocked by what they had just seen and ultimately scaled a fence in their wake as they fanned out in search of clues.
And after realizing that he needed to report the day's events to NBC, his need for a phone took him by chance to the Texas School Book Depository, which the Warren Commission that investigated Kennedy's death concluded was the site from which Oswald fired three shots.
While there, MacNeil had an encounter with a man who may well have been the assassin himself.
"As I ran up the steps, this young guy in shirt sleeves came out. I said, 'where is there a phone?' He said, 'you better ask inside," MacNeil said. "I didn't register his face because I was obsessed with finding a phone. . . . Much later, it occurred to me that I was going in just about the time Oswald had been going out."
Historian William Manchester later reported that Oswald recalled speaking to a man who matched MacNeil's description about a phone. Manchester relayed this to MacNeil, saying the assassin had mistaken the journalist for a secret service man.
But other historians differ on whether MacNeil and Oswald did interact, and the reporter himself said the incident was "titillating but not as important" as the rest of the drama that unfolded that day.
That drama kicked into high gear for MacNeil after he emerged from the book depository and came across police officers at the scene. He was on hand to hear two witnesses _ a man and a child _ say they had seen a gun protruding from one of the depository's windows.
Yet another exchange with law enforcement helped redirect MacNeil back to the heart of the action.
"While I was talking to the policeman, a woman said, '(Kennedy) wasn't hurt was he?' And he said, 'yes ma'am, he was hurt bad.' And he took me over to his motorcycle where the radio was playing and talking about head wounds at Parkland Hospital."
MacNeil beat a hasty path to the hospital where he was able to secure a room with a telephone well ahead of the frantic press core that descended on the building shortly after his arrival.
From his vantage point near the nurse's station, he was able to get frequent updates as the action unfolded and ultimately was the one to inform the NBC editors that the president was dead.
Despite his trailblazing work on the story, MacNeil still has regrets about his approach to the coverage and can't help but dwell on the questions he didn't ask.
"I ran around people lying on the ground covering their kids with their bodies," he said. "Why didn't I stop and talk to them as people later did? They were a few feet away from seeing the president have the top of his head blown off."
The career-making story also had an emotional impact on MacNeil, who found himself weeping helplessly days later as he listened to funeral coverage on the radio.
The assassination left him briefly questioning the wisdom of settling in the United States, he said, adding the story had particular resonance since his kids were exactly the same age as the Kennedy children.
He found himself reflecting on the differences between the U.S. and his home country, he said.
MacNeil was born in Montreal but called Halifax home growing up. He was educated at Ottawa’s Carleton University and worked a stint at CBC before departing for London.
"I had just moved my children from London to Washington, and in my emotional reaction several days after the shooting, I thought, 'what kind of a country have I brought my children to?'" he said. ". . . I couldn't imagine that happening in Canada. I got quite depressed for a few days."
The emotional fog eventually lifted, but the echoes of those fatal shots reverberated throughout MacNeil's 40-year career which ended with his retirement i 1995.
None of the subsequent stories he would cover for NBC, BBC or PBS, he said, could come close to having the impact of the Kennedy assassination.
"Everybody around the world who was of age seems to remember exactly where he was or what he was doing when he heard that news. It was the kind of news that penetrates through the membrane we all wear around us, and only the most piercing, personal information gets through that," he said. "This was that kind of story."