Henri van Breda, 23, had denied murdering his 21-year-old brother Rudi and parents Martin, 54, and Teresa, 55, and leaving his sister Marli struggling with near-fatal injuries to her head, neck and throat.
During the trial, he told the court that a late-night intruder had entered the family's luxury residence on the highly-guarded De Zalze Golf Estate in Stellenbosch, outside Cape Town.
But Judge Siraj Desai systematically took apart the defence case in a five-hour summary of the trial -- and then delivered a verdict that he said "as a family man, it's difficult" to read out.
"After considering all the evidence, the result is inescapable," Desai said, finding van Breda guilty on three counts of murder and one count of attempted murder, as well as of obstructing justice.
"That the accused was the perpetrator of the crime is the only reasonable conclusion," he said.
"No evidence of a typical house robbery or break-in is evident.
"Four of five members of family found brutally attacked in similar fashion and left for dead... The accused was left standing, having lived through the events."
Van Breda showed little emotion as the verdict was delivered, saying only "I do" to confirm that he understood he was being held in custody until sentencing on June 5.
He was then hand-cuffed from behind and tried to say a few words to his weeping girlfriend before he was led into the basement of the court from the dock via steep stairs.
In his testimony, van Breda claimed he was in the toilet when the masked intruder began attacking his family.
He himself suffered superficial stab wounds which he said were caused while wrestling with the attacker who was trying to slit his throat. He said he then fell unconscious.
Prosecutors insisted the wounds were self-inflicted, and on Monday they called for van Breda to be sentenced to a life sentence for each murder.
- 'Nonsensical' -
"The accused testified that he was unaware of anyone with grudges against the family," Desai said.
"It is nonsensical that an intruder with the intention to steal would go upstairs and start attacking the family."
Desai said the axe used in the attack was kept in the house.
He also questioned why van Breda waited three hours to call security or the emergency services.
"If the accused's intention was to be helped as soon as possible, it does not explain why he first called his (then) girlfriend... several times from his mobile phone," Desai said.
Van Breda's family had moved to Australia many years ago before returning to South Africa in 2014.
- Turned himself in -
Believing he was about to be arrested, van Breda turned himself in at a police station 18 months after the killings.
The trial generated fevered interest over a privileged son allegedly unleashing a savage attack on his family, whose fortune -- estimated at $16 million (13.6 million euros) -- was derived from property.
Van Breda's claim of an intruder had echoes of the defence used by Oscar Pistorius, who said he thought a burglar was hiding in the toilet to explain why he fired four times through the door, killing his girlfriend.
In a recording of van Breda's call to emergency services, he said his family members were "bleeding from the head" before he appeared to giggle briefly.
Wearing a grey suit and navy blazer, Van Breda had struggled to stay awake in the dock on Monday, nodding heavily as the grisly deaths of his family were detailed by the judge.