Built as a borstal, Onley held only young offenders for many years until its role was expanded in 1998 to hold juveniles, including those remanded as well as sentenced from 2001 to 2004. In 2003, the Youth Justice Board removed the juvenile population, who were replaced by sentenced adults in March 2004.
The remaining young adult population were removed in March 2010, and in April 2010 Onley became an adult category C training establishment. The prison has a capacity of 742, and the #1 governor is Craig Clark. The stated role of the establishment is to house a proportion of London prisoners approaching the end of their sentence with a view to accessing Community Rehabilitation Companies (CPA Area 20) (previously known as the probation service) to prepare them for release into their home area.The prison is run by HMPS and is part of the East Midlands region.
A short description of residential units is:
- A, B, C, D, E, G wings – general wings, holding 60 prisoners each; G wing has a safer cell
- F wing – care and support unit, 15 prisoners
- H wing – first night and induction, including crisis suite and safer cell, 60 prisoners
- I wing – general wing, holding 100 prisoners in double cells
- J, K wings – general wings holding 75 prisoners each, some in double cells
- L wing – enhanced wing holding 72 prisoners, some in double cells