There was no reason for her to have felt a need to guard herself, she said.
"She was in all respects defenceless."
The defence case during the trial had been that he fatally strangled his mother in an instance of manslaughter and then he later hit her with the hammer.
But today Justice Walker said it was reasonable to conclude the jury believed the cause of death had been the hammer blows to her head, due to defensive wounds.
While it was a brutal attack on a vulnerable person there were several personal factors the judge considered when sentencing Marinovich.
Justice Walker said he had no prior convictions, he had not planned the attack and was genuinely remorseful.
Also, it was only after his arrest that he was diagnosed with autistic spectrum disorder (ASD) and its particular characteristics had played a "significant role".
His thinking is rigid and linear without cognitive flexibility, the court heard.
Marinovich had been struggling to manage as a carer in the time leading up to the murder.
His mother who had bipolar disorder was incontinent, suffered from angina and was unsteady on her feet - often collapsing.
Justice Walker said it would be "manifestly unjust" to impose the 17-year minimum non-parole period required by law for crimes which show a high level of brutality and callousness.
Instead, she sentenced him to a minimum non-parole period of 14 years.
February 8, 2019
Marinovich was at his local train station when he dialled 111 shortly after midnight on the morning of February 8.
He had already driven past the Henderson police station when he pulled over.
"I think I've killed my mother," he told the call-taker.
"I snapped and I hit her in the head with a hammer.