Robert Owens, 47, launched the attack after rowing with his 75-year-old mother Iris Owens at their family home in Ystrad Mynach, Caerphilly county.
Cardiff Crown Court was told the retired history lecturer sustained five separate contacts with the chainsaw and was also strangled and kicked.
Owens has pleaded guilty to murder and will be sentenced on Tuesday.
Judge Mrs Justice Nicola Davies was told Owens had been chopping firewood in May this year when he turned on his mother who had returned home from church.
“Neighbour David Harding heard the sound of someone calling out and then the sound of a chainsaw,” said prosecutor Christopher Clee QC.
“Robert Owens then rang for an ambulance. During the course of that 999 call he said: ‘My mother went mad. I was chainsawing some wood and my mother went mad. I’ve got to give her the kiss of life’.”
Mrs Owens was described as a “very well-educated” woman who had achieved a first-class degree in English when she was 60 years old, was a published author, volunteered for several charities and was a bilingual tour guide at a local historic house.
The court heard that, after the 999 call, two paramedics visited the house and were led to the garden by Owens.
Mr Clee said: “Iris Owens was lying on her back. She had a clear laceration to the left side of her neck, to the skull and mouth area.
“Various statements were made by Owens at the scene. He told the paramedic: ‘My mother had gone berserk. I killed her and I’m going to jail’.”
He said Owens also told a police officer: “I was chopping wood and she just went ballistic on me. She pushed me and I pushed her back. Why did this have to happen?”
Owens also accepted strangling his mother and breaking her ribs by kicking her.
“The precise sequence won’t ever be known,” Mr Clee said.
The court was told the killer was “very close” to his mother and moved in to the home when he went through a divorce in 2006.
“He accepts he used the chainsaw to strike his mother,” said Simon Laws QC, defending Owens.
“There’s no attempt to minimise what he did, he is responsible for these injuries.
“He is devastated by what happened in those few moments of anger in the garden and must live with the consequences.”
The court heard traces of heroin and cocaine were found in Owens’ blood after the murder.
Judge Davies described it as a “tragic and unusual case”.