The Priory healthcare group has been fined £300,000 over the death of a child at one of its hospitals.
Amy El-Keria, 14, was found hanged in her room at the Priory in Ticehurst, East Sussex, in November 2012.
The private company, which runs mental health services as part of an NHS contract, was sentenced at Lewes Crown Court.
In 2016, an inquest found her death may have been prevented if she had received proper care.
The Health and Safety Executive pursued a criminal investigation and the company admitted to a charge of being an employer failing to discharge its duty to ensure people were not exposed to health and safety risks.
In sentencing, Judge Mr Justice James Dingemans said no financial penalty he could impose would ever “reflect the loss suffered by Amy’s family”.
Speaking outside court, Amy’s mother Tania El-Keria said: “The public’s eye has been firmly opened to what the Priory stand for, profit over safety.
“Today is a historic day in our fight for justice for Amy.
“Our Amy died in what we know to be a criminally unsafe hospital being run by the Priory.”
A court hearing in January was told the teenager, who was deemed high-risk and had a “known and recent history” of suicide attempts, was admitted to the hospital on 23 August 2012.
She was left with unsupervised access and the means to carry out another suicide attempt.
On November 12, she was found with a ligature around her neck and taken to Conquest Hospital in Hastings, where she died the next day.
Amy’s mother Tania El-Keria told the court the “nightmare” of losing her “spirited” daughter left her feeling like her “heart and soul is ripped out every morning”.
She admitted to having “low points where I have not wanted to be alive any more just so that I could be with Amy”.
Ms El-Keria added: “I hope that the knowledge gained from this case goes on to change what I see as a failing system and prevents future avoidable deaths.”
Judge Dingemans added: “It is apparent from the investigations that have been carried out in Amy’s death, and the works carried out by Priory Healthcare and the CQC [Care Quality Commission], that there is now a much better understanding of young person suicide, and that vital lessons have been learned.”
Amy had a history of suicide attempts and had a complex range of problems and mental health diagnoses.
She was sent to the Priory after being asked to leave her specialist boarding school in Berkshire.
The inquest into her death heard staff at the unit had not been trained in resuscitation and did not called 999 quickly enough.
The jury said Amy died of unintended consequences of a deliberate act, contributed to by neglect, and that staffing levels at the Ticehurst centre were inadequate.
When imposing the fine, the judge said he took into account the company’s “good” health and safety record, guilty plea and steps made to improve the service.
Priory Healthcare had a turnover of £133m in 2017, with an operating profit of £2m, he said.
It must also pay the Health and Safety Executive’s costs of £65,800 and a victim surcharge of £120.
In a statement, Priory Healthcare said: “The latest CQC report, published in January this year, rated Ticehurst as “good” in all areas.
“We remain absolutely focussed on patient safety and will continue to work closely with commissioners and regulators to learn lessons from incidents and inspections quickly and ensure all concerns are addressed in a timely and robust way. “