Opened in 1960 as a borstal, Guys Marsh became a young offender institution in 1984. In 1992, it became a closed establishment and started to accommodate adults becoming an adult male category C in 2008 prison holding prisoners serving life sentences and indeterminate sentences for public protection, as well as those involved in the integrated drug treatment service (IDTS). Guys Marsh was a designated foreign national prisoner centre and a priority prolific offender centre of excellence. The prison is run by HMPS as part of the South West region and has a capacity of 557. The #1 governor is called Paul Millett.
- Anglia wing is a general wing holding 79 prisoners.
- Cambria wing holds up to 66 prisoners, and is set up as part of a progressive system for those who self-isolate, with safe association and exercise, and places in workshop 10 for those who cannot work in the general population. It also holds the majority of the prison’s life-sentenced population.
- Dorset wing is a general wing holding 66 prisoners.
- Fontmell and Jubilee wings are enhanced wings, holding 40 prisoners each.
- Gwent wing is the induction wing holding 80 prisoners.
- Mercia wing holds 76 prisoners and has a non-smoking spur.
- Saxon wing is the integrated substance misuse service wing, providing 60 spaces to those engaging with drug treatment services.
- Wessex wing holds 66 prisoners, including those on the Kainos programme.
- Tarrant is the segregation unit, with 12 standard cells and 2 special accommodation cells.