The Richmond Fellowship, a charity which cared for more than 2,500 vulnerable Scots, was ordered to pay £450,000 for its role in the death of Margaret Glasgow.
The 59-year-old, who had complex care needs, drowned in the bath when water should have been switched off at the mains.
Ms Glasgow, who had severe learning difficulties, moved into Cherry Tree Court in Cambulsang, South Lanarkshire, four weeks before her death on June 10 2016.
Between 1am and 7am, she filled the bath with water without telling her support workers and subsequently drowned.
Water was supposed to be turned off at the mains before she went to bed and equipment to detect movement in the flat was not properly installed, a court heard.
The Richmond Fellowship, which had denied the charges, was convicted by a jury of a “catalogue of failures”, breaching strict health and safety regulations.
Carers failed to have a suitable plan in place to manage Ms Glasgow’s move from the care centre where she previously lived, and failed to ensure support workers who would be caring for her were given an opportunity to have regular visits prior to the move.
They also failed to carry out a suitable and sufficient assessment to identify her specific needs and the measures required to ensure her safety before she moved to her new home.
Reliable safety equipment to detect movement in the flat was not provided, while a check sheet relating to care was also not provided.
A jury also found that too few instructions were provided to workers regarding the location of the water mains and the correct way to turn water off, which was supposed to happen after she went to bed in the evening, and 'not all' support workers were made aware of the precaution.
The care company also failed to ensure that support practitioners were given specific instructions to ensure that they had sufficient knowledge to carry out appropriate checks.
A jury convicted the company of a health and safety breach.
Imposing the fine, to be paid in 28 days, Sheriff Principal Craig Turnbull said: “The jury have found established what can only be described as a catalogue of failures on the part of The Richmond Fellowship.
"Moreover, by their verdict, the jury were satisfied that each of those failures caused or materially contributed to Margaret’s death.”
The Sheriff Principal said that following the tragedy the Health and Safety Executive (HSE) had reviewed new safety precautions.
He added: “There was a wide reaching review that led to the taking of a number of steps to address the issues raised in this tragic case.
"In this regard, I place some weight upon the evidence of the HSE inspector who stated that the measures taken by the Richmond Fellowship allayed any fears she had that such a tragic accident might happen again.
"I have concluded that a headline sentence of a fine of £450,000 is both fair and proportionate and is no more severe than is necessary to achieve the appropriate purposes of sentencing in this case."
Lawyers for The Richmond Fellowship Scotland told the court changes had been implemented following the tragedy.