Background: This study investigated differences in the clinical and demographic characteristics of individuals with intellectual disabilities delayed in assessment and treatment hospitals versus individuals who were not delayed. The study further investigated the clinical outcomes of the individuals whose discharge from the hospital was delayed. Method: This was a cohort study using secondary data collected from patient records. Variables included age, ethnicity, levels of deprivation, areas of origin, diagnoses, and the provision of a care and treatment review. A comparison was made between individuals who became delayed despite being clinically ready for discharge and those who were discharged on time. Findings: Individuals with severe intellectual disabilities were more likely to experience delayed discharge from hospital (p = 0.001). Similarly, people who were admitted to hospitals away from their home areas were also more likely to experience delayed discharge (p < 0.001). There were no significant differences between the delayed and nondelayed groups for other factors such as age, deprivation or ethnicity. Individuals who experienced a delay in discharge did not experience an increased rate of adverse incidents compared to the period of active treatment and clinical improvements were maintained during the period of delay. There were low rates of completion of care and treatment reviews before admission, although people who were delayed were more likely to have received a review during admission (78.8% vs. 27.1%). Conclusion: It is important for clinicians and service development leads to be aware that people who are admitted away from their home area and people with severe intellectual disabilities may be at higher risk of experiencing significantly delayed discharge from the hospital. More broadly, many people remained in hospital for a substantial length of time after being assessed as clinically ready for discharge. Due to the low rates of completion before admission, the expansion of community-based care and treatment reviews should also be explored. While people did not suffer increased rates of adverse incidents during their extended stay, their liberty was still restricted, hence there should be a focus on effective community provision. The current model of relying on the willingness of independent providers to support the care of complex individuals requires significant reform.