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.