He lost out on a suitable placement because a vacancy at an appropriate home was not followed up in time, the Local Government Ombudsman (LGO) said.
As a result he spent 11 months without the support he needed.
Herefordshire council said it had apologised to the man's family and paid Â£2,500 in compensation.
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Social workers had agreed to find a new home in the 'medium term,' but despite the young man's mother pushing the council to find him one, and suggesting a possible placement, it took social workers 11 months to re-house him, the LGO said.
Dr Jane Martin said the council recognised "it should have done more to proactively find" the man a suitable care placement.
The council said it accepted all its recommendations to remedy the matter, including training for social workers on the importance of keeping accurate case notes.
An autistic man was left for nearly a year in an unsuitable care home because social workers delayed finding him appropriate care.