Lorna Stewart, 35, from Darvel in East Ayrshire, worked as a carer for 16 years before she targeted her 83-year-old victim.
She was caught after suspicious relatives installed CCTV and filmed her stealing the cash.
Sheriff Alistair Watson told Stewart a custodial sentence was "looming large".
Claire McEvinney, prosecuting, said the elderly victim received a pension that was delivered to her by her son.
She added: "Over the course of several months, the complainer's son noticed money appeared to be missing from his mother's house.
"On one occasion, Â£300 went missing from a purse he had hidden in the kitchen."
Kilmarnock Sheriff Court heard the family fitted CCTV after the incident at the house in Galston, Ayrshire.
Miss McEvinney said two further sums of money went missing, adding: "CCTV showed Stewart removing the money on both occasions."
Stewart admitted stealing Â£1,200 in the course of her employment on various occasions between March and October 2016.
Paul Gallagher, defending, said she went to the police and was cautioned and charged but made no reply.
Sheriff Watson said: "But still despite being caught red-handed she decided to plead not guilty initially."
Mr Gallagher replied: "It's clearly a matter of much gravity with all options open, and that has not been lost on her."
Stewart had been working for contractor Constance Care on behalf of East Ayrshire Council but was "no longer in that position", Mr Gallagher added.
Sheriff Watson described the offence, against a pensioner with Alzeimer's disease, as a "despicable and disgusting course of behaviour".
The crime was "calculated and planned", with Stewart selecting a vulnerable victim, he said.
Sheriff Watson added: "It's difficult to understand how anyone would stoop so low.
"I give you warning now that a custodial sentence is looming large and you may wish to prepare yourself for what will follow."
Sentence was deferred until a later date and Stewart's bail was continued.
This story initially featured a photograph provided to us which was wrongly believed to be of Lorna Stewart. However, the image showed someone who was in no way involved in the case. The picture was removed from the story.
A carer who stole the life savings of a woman with Alzheimer's disease has been told to expect prison for her "despicable and disgusting" crime.