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 396. The #1 governor is called James Lucas, who has been in post since February 2017
- Anglia wing is being refurbished and when open will hold 70.
- 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, It also holds the majority of the prison’s life-sentenced population.
- Dorset wing is being refurbished and when open will hold 66.
- Fontmell and Jubilee wings are enhanced wings, holding 40 and 60 prisoners respectively.
- Gwent wing holds 75 prisoners.
- Mercia wing holds 80 including induction.
- Saxon wing holds 75 men.
- Tarrant is the segregation unit, with 12 standard cells and 2 special accommodation cells.