The 16-year-old girl was arrested in November and later sectioned. But, with no bed available, she spent two days in a cell at Torquay police station.
Her case sparked an outcry and debates in Parliament, with the home secretary vowing to ban the practice.
NHS England has since apologised and described her care as "unacceptable".
The girl's mother, who is not being named to protect the girl's privacy, told BBC Inside Out South West she was "absolutely destroyed" to find her daughter sleeping in a cell.
"When I walked in and saw her lying there, on that floor... heartbreaking," she said.
"She was on a blue thing on the floor - it's not even what you class as a police bed.
"She needed the right help. The police, I can't fault them, they did an amazing job looking after my child."
More than 24 hours after the girl was put in the cell, Paul Netherton, Assistant Chief Constable at Devon and Cornwall Police, took to Twitter to express his frustration and voice concerns for her welfare.
His comments provoked a national debate and questions in Parliament. Health Secretary Jeremy Hunt said it was "totally unacceptable for someone with severe mental health problems to be placed in a police cell".
The girl was arrested on Thursday evening at Torbay Hospital for an alleged breach of peace. She was then taken to Torquay police station where she was detained under the Mental Health Act on Friday morning. She was never charged with any offence.
Once someone is detained under the act, it is the responsibility of the NHS to find a place in a specialist hospital where appropriate care can be provided.
As no bed was made available, the girl had to remain in a police cell until Saturday evening.
However, Exeter Labour MP Ben Bradshaw has claimed a bed was available, but the private provider declined the referral.
NHS England would not comment on the specific case, but said providers "sometimes decline to accept referrals if they feel they cannot meet the individual's needs".
Source: Devon and Cornwall Police
The girl's mother praised the intervention of Mr Netherton and said that evening - two days after the teenager was first placed in the cell - a bed on an adult psychiatric ward was found.
"My daughter shouldn't have been held in a police cell for that long," she added.
In a joint statement, the South Devon and Torbay Clinical Commissioning Group, NHS England and Torbay and Southern Devon Health and Care NHS Trust, said the care was "unacceptable and we apologise".
"Following a review of this case, we have agreed a system to ensure that, in any case where a child cannot be found an appropriate placement rapidly, a clear escalation process will be followed," the statement added.
However, the mother said she was yet to receive an apology from the NHS.
The girl has now been moved to a specialist hospital more than 200 miles (320km) from her family home.
"My daughter needs her family. We need her, she needs us. We are a close family and we don't cope without each other," her mother added.
"The fact she's so far away from all of us hurts her even more."
Following the case, the government announced the use of police cells as a "place of safety" for teenagers experiencing mental health problems would be banned.
Home Secretary Theresa May said: "There is no place for this in our society."
Inside Out South West is broadcast on BBC One on Monday, 26 January at 19:30 GMT and nationwide on the iPlayer for 30 days thereafter.

It has caused big problems for people living in parts of northern England and Scotland.
Heavy rain and high-speed winds have resulted in water flooding roads and houses.
Hundreds of people have been forced to leave their homes as rivers burst their banks and flood waters rose quickly.
Around one month's worth of rainfall fell in just 24 hours.
There are more than 50 flood warnings in place across the UK and the Met Office has advised people not to travel.
Dozens of trains have been cancelled, and roads have been closed in Scotland, England and Wales.
A mentally ill teenager who was held in a cell for two days due to a lack of hospital beds suffered "heartbreaking" failings in her care, her mother says.
[NEXT_CONCEPT]
Schools and hospital services have been closed in parts of northern England, as emergency services continue to battle against the effects of Storm Desmond.