A man has pleaded guilty to manslaughter over the 1988 death of American mathematician Scott Johnson.
Scott Phillip White, 52, was arrested in 2020 almost 40 years after Dr Johnson's body was found at the bottom of a cliff at a Manly gay beat.
He pleaded not guilty on Thursday in the NSW Supreme Court to murder but guilty to manslaughter, accepting legal responsibility for the death.
The guilty plea represents the end of a very long saga for the Johnson family, Detective Chief Inspector Peter Yeomans said outside court.
"Today really vindicates the Johnson family, what they've done over many, many years," he said.
"They fought for justice and (it's) finally come to fruition."
White previously maintained his innocence, having reversed a guilty plea to murder made at a pre-trial hearing.
He 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.
The judge who heard the plea in January, 2022 refused to allow White to reverse it and later sentenced him to at least eight years and three months in jail.
The conviction and jail term were quashed when the NSW Court of Criminal Appeal in November found the judge had erred for legal reasons in refusing to allow White to withdraw the plea.
The primary judge used a "miscarriage of justice" test when the "interest of justice" test, which is broader, was appropriate, the appeal court ruled.
The mystery of Scott Johnson's death has been subject to three inquests with varying findings.
Dr Johnson worked for NASA's Jet Propulsion Laboratory mapping the surface of Venus before moving to Australia in May 1986. He was working at the Australian National University at the time of his death.
His death was initially ruled a suicide, only for the case to be reopened in 2012 after pressure from his family.
A coroner in 2017 found Dr Johnson was violently attacked by someone who perceived him to be gay.
However, Justice Helen Wilson found there was not enough evidence to show the murder was a gay hate crime.
A $2 million reward for information was also made by NSW Police and his family.
White will return to court on June 6 for submissions on sentence.
