Background: The Manchester Attachment Scale-Third party observational measure (MAST) was developed to assess secure attachment style for adults with intellectual disabilities. The psychometric properties of the MAST were examined. Materials and Methods: Professional carers (N = 40) completed the MAST and measures related to the construct of attachment theory [Edward Zigler-Yale Personality Questionnaire (EZPQ), Emotional Rating Scale (ERS) and the Learning Disability Casemix Scale (LDCS)] regarding individuals with an intellectual disability (N = 57). Individuals with an intellectual disability (N = 14) completed the Self-report Assessment of Attachment Security (SRAAS). Results: The MAST was found to have good internal consistency, test-retest reliability and convergent validity. MAST scores were negatively correlated with level of intellectual disability and challenging behaviour (CB) as measured by LDCS. Conclusions: Support was provided for the reliability and validity of the MAST and a relationship between attachment security, level of intellectual disability and CB. The results of the study and implications of attachment theory for service provision are discussed.