Social services documents requested by a family member from Wokingham Borough Council went missing after the delivery driver left them outside the requester's home in August 2013.
They included details of allegations of neglect and abuse carried out by the requester's ex-partner.
The council has agreed to take action.
The Information Commissioner's Office (ICO), which carried out the investigation, said the driver had not been told about the sensitivity of the information, or that the delivery required a signature or returning to the council if no-one was available.
The council had also failed to arrange a suitable delivery time with the requester, it added.
Stephen Eckersley, ICO's head of enforcement, said a "series of errors" by the council led to a social services record being delivered "with no consideration given to its content".
The council has agreed to ensure future deliveries containing sensitive personal information are carried out securely.
It said it would also make sure its staff receive regular training in data protection and information security.
A council breached the Data Protection Act after sensitive records relating to the care of a young child were lost, an investigation has ruled.