Intellectual Disability is under-ascertained worldwide and is associated with greater physical and mental health difficulties. This research aimed to identify clinical features and characteristics of children with Intellectual Disability in a population of 126 6–18 year olds in mainstream school, attending paediatric developmental clinics. Intellectual Disability was defined according to the DSM-5 (deficits in intellectual and adaptive functioning, present during childhood). Measures used to assess this were WISC-IV IQ (score <70) and ABAS adaptive behaviour (score =<70). Clinical features were compared from a structured clinical records investigation and logistic regression explored which factors were associated with Intellectual Disability. Twenty-eight children (22%) met the criteria for Intellectual Disability. Five variables were associated with higher odds of having Intellectual Disability: no other neurodevelopmental diagnosis, multiple other health problems, prior genetic testing, maternal smoking during pregnancy, and parental unemployment. Routinely-collected paediatric data only predicted Intellectual Disability correctly in two out of five cases. Further research is needed to verify these findings and improve identification.