Expansion of the youth mental health sector has exposed a need for an outcome measure for young adults accessing services. The Strengths and Difficulties Questionnaire (SDQ) is a widely used consumer and carer outcome measure for children and adolescents. The aim of this study was to evaluate the psychometric properties of a young adult SDQ. The young adult SDQ was introduced for routine clinical practice at Eastern Health Child and Youth Mental Health Service (EH-CYMHS), complementing the well-established adolescent and child versions. Data for adolescents (aged 12–17) and young adults (aged 18–25) where both self-report and parent SDQs had been completed at entry point to the service were extracted from a two-year period. Overall, paired cases involved 532 adolescents and 125 young adults. Across both self-report and parent SDQs, a similar pattern of results was found between adolescents and young adults on mean scores, inter-scale correlations, internal consistency, and inter-rater agreement. The findings of the current study support the use of the young adult SDQ in public mental health as an instrument whose psychometric properties, to date, appear consistent with those of the adolescent version. Further investigation is warranted.