Objective: Interorganizational relationships (IORs) between child welfare agencies and mental health service providers may facilitate mental health treatment access for vulnerable children. This study investigates whether IORs are associated with greater use of mental health services and improvement in mental health status for children served by the child welfare system. Methods: This was a longitudinal analysis of data from a 36-month period in the National Survey of Child and Adolescent Well-Being (NSCAW). The sample consisted of 1,613 children within 75 child welfare agencies who were 2 years or older and had mental health problems at baseline. IOR intensity was measured as the number of coordination approaches between each child welfare agency and mental health service providers. Separate weighted multilevel logistic regression models tested associations between IORs and service use and outcomes, respectively. Results: Agency-level factors accounted for 9% of the variance in the probability of service use and 12% of mental health improvement. Greater intensity of IORs was associated with higher likelihood of both service use and mental health improvement. Conclusions: Having greater numbers of ties with mental health providers may help child welfare agencies improve children's mental health service access and outcomes. Practice implications: Policymakers should develop policies and initiatives to encourage a combination of different types of organizational ties between child welfare and mental health systems. For instance, information sharing at the agency level in addition to coordination at the case level may improve the coordination necessary to serve these vulnerable children. (Contains 1 figure and 4 tables.)