The purpose of this study is to test the validity of the Assessment Checklist measures in assessing complex mental health and behavioural difficulties of children and young people in care attending a specialist mental health service in Queensland, Australia. Fifty-eight consumers (53% male) with an average age of 8 years were assessed by carers on the Assessment Checklist for Children—Short Form and Strengths and Difficulties Questionnaire, and 44 consumers (36% male) with an average age of 13 years were assessed by carers on the Assessment Checklist for Adolescents—Short Form and Strengths and Difficulties Questionnaire. Results showed that the Assessment Checklist for Children—Short Form total score correlated with the Strengths and Difficulties Questionnaire total score. There were some correlations between subscales on both the measures. The Assessment Checklist for Adolescents—Short Form and Strengths and Difficulties Questionnaire correlated on very few domains. Domains of the Assessment Checklist measures that assess emotional dysregulation, trauma, interpersonal/attachment styles, sexual behaviour and food maintenance appear to provide additional clinical information about consumers that the standard Child and Youth Mental Health Service carer-report measure (Strengths and Difficulties Questionnaire) does not. It is recommended that the Assessment Checklist measures be used as an additional measure to assess the complexity of the children and young people in care who attend specialist mental health services.