The 19-year-old, who cannot be named, was held in care by Somerset County Council.
The Court of Protection said the lack of a "proper investigation" showed a "systematic failure" by the council.
The council said it had apologised to the family for the distress caused.
At a hearing at the court in Bristol, Judge Nicholas Marston said staff had failed to respect the human rights of the woman and her family, who come from Yeovil.
The court was told that just before a period of respite care in May 2013, the woman's mother noticed bruising on her daughter's chest and contacted a doctor and respite care staff.
But after her admission staff concluded her injuries were unlikely to have been self-inflicted as no self-harm had been witnessed.
Based on medical report findings that it was "highly likely" the woman had received a "significant injury from someone or something other than herself", social workers decided she could not return home.
But a day before her mother spotted bruising, the judge said staff at her specialist school saw her "hitting herself on the sternum area".
Judge Marston said this information would have been "easily discoverable" if social workers had carried out a "proper investigation".
A spokesman for Somerset County Council said it accepted the court's ruling but it had been "completely motivated by serious concern for the young lady's welfare".
"We are working closely with them [the family] to provide the right care and support for their daughter now and in the future," he said.
A disabled woman who was prevented from going home for a year after care staff mistook self-inflicted bruising for abuse, was "removed unlawfully" from her family, a judge has ruled.