Medical Records in Ontario are held in various places - some are at the Ontario Archives, others are with the original institution.
To obtain these records, you should begin with the OA. Find out what they have, and then how to request it. For example,Queen Street Hospital in Toronto has records that go back to 1839. There are nineteenth century records for institutions in London, Hamilton, and Kingston and from Penetanguishene from 1904.
Files less than 100 years old are restricted under the Freedom of Information and Protection of Privacy Act. Requests for access must be submitted in writing to the
Access Unit of the Archives of Ontario.
Files created in whole or in part on or after 6 April 1954 are restricted under the Mental Health Act. Requests for access to records in Archives' custody must be submitted in writing to the Administrator of the appropriate psychiatric hospital.
Olive Tree Genealogy has a partial list of the OA psychiatric files and holdings