The Working Alliance Inventory remains a widely studied measure of quality of therapeutic relationships between the practitioner and client. No prior study has examined the psychometrics and validity of the Working Alliance Inventory–Short (WAI-S) in a sample of families, social workers, and trained observers within child protection services. Surveys were completed by 130 families, social workers concerning 274 cases, and observers following 165 home visits during the first wave of data collected from a randomized controlled trial of child protection services. Confirmatory factor analyses were conducted on three versions of the WAI-S and demonstrated moderate to good model fit. Convergent construct validity was found with other standardized measures. Results support the use of the WAI-S during in child protection services practice and research. Future research into family engagement in child protection social work services should focus on the working relationship.