Robert Owens, 47, attacked his elderly mother - Iris - in the garden of the family home as she hung her washing up to dry. Iris, 75, died from her injuries.
The brutal attack happened in May and Owens pleaded guilty to murder at a hearing in September.
Prosecutor Christopher Clee QC said the attack took place at the home the pair shared in Ystrad Mynach, near Caerphilly, Wales.
"He killed her with a chainsaw," he said. "The cause of death was trauma to the head and neck.
"The precise sequence is unclear but at 5.21pm the defendant rang for an ambulance," Clee told the court.
In the 999 call, Owens said: "Yeah, my mother just went mad. She's in the garden now, please help her."
Paramedics found Mrs Owens lying on her back near a tree stump in the rear garden, with a cut to the left side of her neck. Mr Owens was reported as being covered in blood and in an "agitated" state.
Tests later revealed morphine, heroin and cocaine in his system. Owens admitted to the police he was a heroin addict.
Clee told the court that Mrs Owens as an "active and independent lady" who volunteered for charities and attended church. Mr Owens moved back in with his mother after separating from his wife in 2006.
Representing Owens, Simon Laws QC, said his client was filled with regret.
"This was a close, loving and supportive relationship so to sit where he does today having done what he did is a very terrible thing," Laws said.
"He would like it said on his behalf that there's not a moment of any day when he is not reminded of what happened.
"He is devastated by what happened in those few minutes of anger in the garden."
Owens will be sentenced on Tuesday.