The American mathematician had received exciting news about a problem he'd been labouring over just hours before falling to his death off a Sydney cliff in 1988.
Three decades on, the man who punched the gay 27-year-old causing him to stagger and fall has been sentenced to nine years in jail.
With a non-parole period of six years, Scott Phillip White, 52, will be eligible for release in 2026 taking into account time already served, though Dr Johnson's family feels vindicated.
"We know it was (White), he's confessed to doing it, but I wish we knew more about why," Dr Johnson's brother, Steve, said outside the Supreme Court on Thursday.
"But I think our family finally has some peace."
Steve Johnson said his brother was "like a best friend" and "had a lot to be proud of" including his brilliant mathematic brain.
"Scott was in a class of his own," he said.
"He died just a couple of years before the internet came along. He would have been the king of geeks."
During White's sentencing, Justice Robert Beech-Jones said the circumstances surrounding Dr Johnson's death could remain a mystery for some time.
"Not much is known about the killing of Dr Johnson beyond a punch near a cliff, a vulnerable victim, a fall over the cliff, a death, an absence of taking even the simplest step to render help after the fall and decades of pain and grief that followed," Justice Robert Beech-Jones said.
On December 10, 1988, White met Dr Johnson at a hotel and the pair went for a walk around Manly's North Head which was known at the time to be a gay beat.
White, then 18, threw a punch at Dr Johnson in the heat of an argument causing the 27-year-old to stagger back over the cliff.
The American was close to receiving his doctorate from Australian National University, which he has since been awarded posthumously.
"Dr Johnson was an American citizen ... he had everything to live for," Justice Beech-Jones said.
"The offender left (him) to die."
White was jailed for 12 years and seven months in 2022 after making a startling guilty plea to murdering Dr Johnson.
The accused killer successfully overturned his guilty plea and conviction in November before pleading guilty to the lesser charge of manslaughter in February.
The 52-year-old had long denied murdering Dr Johnson, only to say "guilty, I am guilty" during a pre-trial hearing in January 2022.
He later told his confused lawyers at the time "I didn't do it but I'm saying I'm doing it ... it's the only way, she's going to come after me" - suggesting his former wife would attack him.
White, who has early onset dementia due to excessive alcohol abuse, was described as a Manly "'street kid' at the time of the killing.
"The offender was clearly a damaged albeit physically powerful young man," Justice Beech-Jones said.
"However, he was not broken as he is now."
Originally thought of as a suicide, police eventually opened an investigation into what they thought was a gay hate crime in 2012.
When initially interviewed by police, White denied that he chose gay victims to bash and told the police, "I'm gay myself".
In her now overturned judgment on the murder conviction, Justice Helen Wilson found there was not enough evidence to show the attack was motivated by Dr Johnson's sexuality.
Justice Beech-Jones said he could not be satisfied beyond reasonable doubt that the crime was a "gay hate crime".
"The answers to numerous other questions about how Dr Johnson died, why he died and what happened in the long decades between his death and today are not yet known," Justice Beech-Jones said.
"Some of those answers may never be provided."
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