Opened in 1960 as a borstal, Guys Marsh became a Young Offender's Institution (YOI) in 1984. After completion of perimeter fencing in 1992 it became a closed establishment and started to accommodate adults. It is now a category C prison. Guys Marsh has constantly changed and grown from a YOI holding 300 and within 18 months it has become a prison holding up to 578.
There are currently 8 living units: 6 units holding 498 prisoners, 1 unit holding 40 enhanced prisoners and 1 unit in a less secure setting holding 40 enhanced prisoners.

