Background Parental mental illness is a major public health issue and there is growing evidence that family focused practice can improve outcomes for parents and their families. However, few reliable and valid instruments measure mental health and social care professionals’ family focused practice. Objectives To explore the psychometric properties of the Family Focused Mental Health Practice Questionnaire in a population of health and social care professionals. Methods Health and Social Care Professionals (n = 836) in Northern Ireland completed an adapted version of the Family Focused Mental Health Practice Questionnaire. Exploratory factor analysis was used to test the structure of the underlying dimensions in the questionnaire. The results, and theoretical considerations, guided construction of a model that could explain variation in respondents’ items. This model was then validated using confirmatory factor analysis. Results Exploratory factor analysis revealed that solutions including 12 to 16 factors provided a good fit to the data and indicated underlying factors that could be meaningfully interpreted in line with existing literature. From these exploratory analyses, we derived a model that included 14 factors and tested this model with Confirmatory Factor Analysis. The results suggested 12 factors that summarized 46 items that were most optimal in reflecting family focused behaviours and professional and organizational factors. The 12 dimensions identified were meaningful and consistent with substantive theories: furthermore, their inter-correlations were consistent with known professional and organizational processes known to promote or hinder family focused practice. Conclusion This psychometric evaluation reveals that the scale provides a meaningful measure of professionals’ family focused practice within adult mental health and children’s services, and the factors that hinder and enable practice in this area. The findings, therefore, support the use of this measure to benchmark and further develop family focused practice in both adult mental health and children’s services.