The conduct of outcomes research on clients with serious mental illness using community mental health services is a challenge. Causal models with inclusion of mediating and moderating variables from social sciences evaluation methods provide a framework for conceptualizing and evaluating the complexity of community mental health services. This article presents the conceptualization and development of a framework for comprehensive evaluation of client outcomes in community mental health services and describes a case example of operationalizing and testing the framework in an evaluation of Assertive Community Treatment (ACT) in Southwestern Ontario, Canada. The initial framework was developed by hypothesizing a cause-effect pathway and links among delivered treatment variables, the implementation system, external factors, and intermediate and longer term outcomes. The framework was further validated and modified through stakeholder input. All variables identified in the framework were then operationally defined and instruments with good psychometric properties were chosen to measure the variables. This framework can provide a generic example for the conduct of community mental health evaluations.