A homeless 17-year-old boy was found emaciated, after having to live in a tent for five weeks, bought for him by Cornwall Council.
After approaching the local authority for help, the "vulnerable" teenager was offered supported accommodation but turned it down.
A social worker then bought the boy a tent to live in and even helped him pitch it.
He spent five weeks living under canvas and was even offered a replacement by the Council when the first one leaked.
The teenager, who has not been identified, had a history of cannabis use and suffered mental health problems.
He had to be detained in a psychiatric hospital for 11 months following his ordeal in 2016.
An investigation by the Local Government and Social Care Ombudsman has found numerous failings in the boy's care by the local authority.
The council said no other options were available because he refused an offer of supported accommodation 30 miles away and did not want to go into care.
However, the inquiry suggested that the local authority tried to place responsibility on the boy, rather than provide the right support for him.
Records show the boy's mother, who lived a long way from Cornwall,challenged the decision to place her son in a tent.
Around a month later, the council moved him to a bed and breakfast, which is against government guidance for housing teenagers.
Shortly after he was moved, he was detained under the Mental Health Act.
Michael King, the Local Government and Social Care Ombudsman, said: "There is a long list of failures in this case which had dreadful consequences for the boy.
"It is true the boy in this case showed difficult behaviours. However, this isexactly why the Children Act exists - to support the most vulnerable in oursociety - and councils should not apportion blame when help is needed."
The ombudsman ordered the council to pay the teenager £2,500 and his mother £1,500.
In a statement, Cornwall Council says it accepts the report and its findings and they have apologised to Mr B and to his mother for those failings.
"Although this was a unique and exceptional case, we will learn from it and do everything we can to prevent it ever happening again."