Introduction: The aim of this study was to examine factors associated with length of stay within a psychiatric hospital for patients with intellectual disabilities who have a forensic history. Methods: Data about 111 patients were gathered retrospectively from historical records for the period of February 2011 to March 2021. Negative binomial regression was then used to examine the relationship between selected predictor variables and length of stay. Results: Patients who were older upon admission and those who had received psychological therapies or positive behavioural support (PBS) had a significantly longer length of stay. Those with a diagnosis of a neurodevelopmental disorder had a significantly shorter length of stay. All other predictors were not statistically significant. Conclusions: There was evidence of a clinical improvement at discharge and those with autism or ADHD had a shorter length of stay. Similar studies with larger sample sizes should be completed across England.