Where can I get prison records from Mimico Reformatory in the 1950's?

Try the Archives of Ontario in the Preliminary Inventory of the Ministry of Correctional Services (RG20) area. There you can find jail files for inmates 1939-1957... also probation & parole files, adult & juvenile case files, records for industrial schools, jails, detention centres and industrial farms. There is a 30 year restriction on personal information and some files may be closed altogether.