Healthcare organisations in the USA rank significantly lower in quality of care compared with other developed nations. Research shows US performance emphasises expensive treatment over effective prevention programmes. This study demonstrates how a comprehensive quality improvement programme can improve health outcomes in a large county-based Medicaid health plan. The health plan serves a diverse community of members spanning racial and ethnic groups with varying levels of clinical risk and social determinants of health burdens. We used a regression discontinuity design to evaluate the impact of a comprehensive quality improvement programme vs using mainly pay-for-performance on Healthcare Effectiveness Data and Information Set (HEDIS) metrics over the course of 10 years. We found significant improvements in several HEDIS metrics that occurred after the quality improvement programme was implemented. These results demonstrate the importance of using a comprehensive quality improvement strategy along with pay-for-performance to improve health outcomes. It was determined that this research was exempt from institutional review board approval, as it used administrative healthcare data, and did not involve direct interventions with human subjects.