Thorn Cross opened in December 1985 as an open youth custody centre on a former Royal Naval air station site that had been converted into an open prison for adults after the war. It operated regimes for young adults aged 18–21 and adults aged 21–25, but this upper age limit was removed in 2013.
- Units 1-5 each unit has 60 single rooms. Unit 4 is currently closed to enable the installation of a new fire alarm system.
- Unit 6 an enhanced unit with 10 single rooms for prisoners who are ready to progress to outside work opportunities. An additional four rooms can be used if a prisoner needs to be held securely before being transferred back to closed conditions.
- Unit 7 has 33 single rooms and is a reverse cohort unit (see Glossary of terms).
- Unit 8 has 44 individual self-contained temporary living units to accommodate prisoners on workouts and those who are shielding or isolating